Diagnostic applications of double D-loop formation

ABSTRACT

The present invention describes the formation of RecA protein catalyzed double-stranded probe:duplex linear target DNA complexes that are stable to deproteinization. The uses of this stable probe:target complex in diagnostic/DNA detection systems in in vitro and in situ DNA hybridization reactions is discussed. The probe:target complexes are also useful for diagnostic application in RecA protein facilitated DNA amplification reactions.

This application is a continuation-in-part of co-pending, co-owned, U.S. application Ser. No. 07/520,321, filed 7 May 1990.

FIELD OF THE INVENTION

The present invention relates to the formation of RecA-catalyzed stable double D-loop structures that can be utilized in a variety of diagnostic methods including two probe capture/detection systems, RecA-facilitated DNA amplification, and in situ hybridization.

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BACKGROUND OF THE INVENTION

RecA+ protein (wild type) is a 37,842 dalton protein found in the bacterium Escherichia coli, which is important for homologous DNA recombination. Most information about its biochemistry and enzymology comes from in vitro studies using purified RecA+ protein. Numerous in vitro studies have shown that RecA+ protein is intimately involved in the pairing reaction between homologous DNA sequences that ultimately leads to homologous recombination events (Radding; see Cox et al. or Roca et al. for recent reviews of RecA+ protein properties). It is this pairing reaction that makes RecA+ protein highly useful for DNA diagnostics applications.

In the presence of ATP, RecA+ protein catalyzes strand exchange between a number of substrates, the most relevant for DNA probe applications being single- and double-stranded DNAs. RecA protein coated single-stranded DNA probes interact with the homologous portion of a double-stranded ("native") target sequence initially by forming a recombination intermediate containing hybridized, partially joined molecules called (single) D-loops (or in some cases triple-stranded structures) (Shibata et al., 1979). This is followed by branch migration, and forming of fully hybrid molecules between the original single- and double-stranded DNAs, depending upon the extent of their homology.

Short displacement loops or triple-stranded D-loop structures in linear targets are usually unstable after deproteinization. RecA protein has been shown to form stable complexes with short oligonucleotides, between 9 and 20 bp (or larger) in length, in the presence of ATPγS and excess RecA protein (Leahy et al.). When linear double-stranded targets are used, stable probe target pairing after RecA removal appears to require (i) a homologous region of at least 38 to 56 bp, and (ii) the location of the probe target homology at the end of the linear duplex (Hsieh et al. 1990; Gonda et al.).

Rigas et al. reported that a single-stranded 43-mer could form a single D-loop complex stable to deproteinization when double-stranded negatively supercoiled circular plasmid DNA was used as the target.

When a double-stranded negatively supercoiled circular target DNA is used, RecA coated single-stranded oligonucleotide probes can also be stabilized by psoralen crosslinking before removal of the RecA protein: probe-target single D-loop products can be recovered if the oligos are at least 30-mer size (Cheng et al., 1989). To obtain psoralen crosslink stabilized single D-loop probe-target complexes when double-stranded linear DNA duplexes are used as target DNA, the probes must be at least 80 to 107-mer size (Cheng et al., 1988): these reactions are very low efficiency when compared to similar reactions with negatively supercoiled circular targets.

Experiments performed in support of the present invention have demonstrated that probe:target DNA complexes, which are stable to deproteinization, can be generated in RecA protein catalyzed reactions providing that duplex probes, which contain sequences complementary between probe strands, are used in the hybridization reactions. This discovery provides a number of opportunities for diagnostic application that exploit this stable RecA protein catalyzed probe:target hybridization complex.

SUMMARY OF THE INVENTION

The present invention includes a diagnostic method for detecting a linear duplex DNA analyte, having first and second strands, where the analyte contains a first internal DNA target sequence. The method teaches providing a set of two DNA probes that each contain sequences complementary to the first target sequence strand or the second target sequence strand, where these probes also have a region of complementary overlap to each other. Both probe strands are then coated with RecA protein in a RecA protein coating reaction. The RecA coated probes are combined with the linear duplex DNA, which contains the target sequence, under conditions that produce a probe:target complex. This probe:target complex contains both probe strands and both strands of the linear duplex analyte: the complex is stable to deproteinization. The presence of the probe DNA in the probe:target complex is then detected.

In one embodiment of the invention the RecA protein is the wild-type RecA protein of Escherichia coli. Alternatively, the RecA protein can be the mutant RecA-803 protein of Escherichia coli or a RecA-like protein from a number of sources.

The RecA-protein coating reactions of the present invention can be carried out using a variety of co-factors, including ATP-γ-S, GTPγS, mixes of ATPγS and rATP, or rATP alone in the presence of a rATP regenerating system.

In one embodiment of the invention, the region of complementary overlap between the probe strands is at least about 78 base pairs and less than about 500 base pairs. The probe strands may also contain an end terminal extension of DNA that is not complementary to either target strand. When both strands contain such an end terminal extension, these DNA extensions may be complementary to each other.

One way in which to accomplish the detecting of the method of the present invention is by deproteinization of the probe:target complex, followed by electrophoretic separation of the probe:target complex from free probe. The probe:target complex can be deproteinized by a variety of methods including treatment with SDS or proteinase K, as well as standard chemical deproteinization methods, such as phenol-based methods. Alternatively, the detecting can include the use of a capture system that traps the probe:target complex, where the first probe strand is labeled with a capture moiety and the second probe strand is labeled with a detection moiety. For example, one probe strand can be biotin labeled and the other digoxigenin labeled. The probe:target complex can then be captured/detected by solid support-streptavidin (or avidin)/labeled anti-digoxigenin, or solid support-anti-digoxigenin antibody/labeled streptavidin (or avidin). In a different embodiment, the first probe strand contains a capture moiety and the second probe strand contains a radioactive label to be used for detection.

The probe strands can be labelled for capture in a number of ways, for example, using biotin or digoxigenin attached to the probe and streptavidin (or avidin) or an anti-digoxigenin antibody, respectively, for capture. The probe strands can also be labelled for detection using a number of different moieties including: radioactive, biotin, digoxigenin. Radioactive labels can be identified by, for example, autoradiography or scintillation counting. The presence of biotin or digoxigenin can be detected by streptavidin or an anti-digoxigenin antibody, respectively, where the streptavidin (or avidin) or anti-digoxigenin is radioactively labeled, enzyme labeled (e.g., alkaline phosphatase, peroxidase, beta-galactosidase or glucose oxidase) or fluorochrome-labeled (e.g., fluorescein, R-phycoerythrin, or rhodamine). Detection of the probe strands in the probe:target complex can also be accomplished by DNA polymerase facilitated primer extension from the 3'-ends of each probe strand, where the primer extension is performed in the presence of all four dNTPs and one or more dNTP contains a detectable moiety.

The method of the present invention further includes providing a second set of two DNA probes, having first and second strands, complementary to a second duplex target sequence, where the first strand of the probe contains sequences complementary to one strand of the second target sequence and the second strand of the probe contains sequences complementary to the other strand of the second target sequence, where (i) these probes also have a region of complementary overlap to each other, and (ii) the second set of probes does not hybridize to the first set of probes. The two probe sets are coated with RecA protein in a RecA protein coating reaction. The RecA coated probe sets are combined with the linear duplex DNA, containing the two target sequences. The combining is done under conditions that produce probe:target complexes which contain all four probe strands. The resulting probe:target complex is stable to deproteinization. The presence of the probe DNA is then detected in the probe:target complexes.

The method involving two probe sets can be utilized in many of the same ways as described above for a single probe set. For example, the first probe set can be labeled with a capture moiety and the second probe set labeled with a detection moiety.

The double-stranded probe:duplex target complexes involving two probe sets can also be used in a RecA protein facilitated DNA amplification method. For example, the two probe sets can be hybridized to their duplex target sequences in the presence of ATPγS and reacted in a reaction mixture also containing, all four dNTPs, RecA protein and DNA polymerase. This reaction is performed below the temperature required for thermal dissociation of the two target strands and continued until a desired degree of amplification of the target sequence is achieved. The amplification reactions may further included repeated additions of (i) DNA polymerase and (ii) RecA protein-coated probes during the course of the amplification reactions. Other approaches to amplification, which can be applied to the present invention, have been set forth in co-pending U.S. application Ser. No. 07/520,321, filed 7 May 1990, herein incorporated by reference. In each probe set, the 3' end of one strand will be internal to the region defined by the two primer sets: these ends are necessary for the amplification reaction. However, the opposite 3' ends of each primer pair, external to the region defined by the two primer sets, can be blocked to inhibit the formation of extension products from these ends. This amplification method can also be used as a detection method, where detection of the probe in the probe:target complex is accomplished by DNA polymerase facilitated primer extension from the 3'-ends of each probe strand, where the primer extension reaction is performed in the presence of all four dNTPs and one or more dNTP contains a detectable moiety.

The double-stranded probe:duplex target complexes can also be used to block cleavage of any targeted restriction site. Blocking cleavage can be accomplished in a number of ways including: (i) forming the probe:target complex and treating with the restriction enzyme before deproteinization of the complex; (ii) using methylated or un-methylated probes, depending on the sensitivity of a selected enzyme to the presence of methyl groups; and (iii) introducing a sequence mismatch in each strand of the probe, which, when the probe hybridizes to the target, eliminates the restriction site.

The double-stranded probe:duplex target complexes can also be used to generate site specific cleavage of double-stranded target DNA. The double-stranded probe can be modified with moieties capable of cleaving each strand of the target duplex: this probe modification can take place before or after deproteinization depending of the nature of the cleaving moiety. Examples of such moieties are iron FeII (for iron/EDTA facilitated cleavage), non-specific phosphodiesterases, and restriction endonucleases. In either case, cleavage specificity is conferred by the target sequence which is defined by the double-stranded oligonucleotide probe.

Both the restriction site protection method and the site specific cleavage method are useful in restriction fragment length polymorphism analysis.

The double-stranded probe:duplex target complexes of the present invention can also be used for diagnostic in situ detection techniques.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the relationships of the probes and primers, listed in Table 1, to the lambda genome.

FIG. 2 presents the nucleotide sequence of a 500 bp lambda genomic region: this sequence is also presented as SEQ ID NO:1.

FIG. 3 shows an autoradiogram of a gel DNA band-shift assay to illustrate RecA protein binding to DNA probes.

FIG. 4A shows an ethidium bromide stained gel on which the components of deproteinized hybridization reactions using 500-mer and 280-mer probes were resolved. FIG. 4B shows an autoradiograph of the gel shown in FIG. 4A.

FIG. 5A shows an ethidium bromide stained gel in which the components of deproteinized hybridization reactions using 280-mer, 121-mer, and 79-mer probes were resolved. FIG. 5B shows an autoradiograph of the gel shown in FIG. 5A.

FIG. 6A shows an ethidium bromide stained gel in which the components of deproteinized hybridization reactions using differentially labeled 121-mer DNA probes were resolved. FIG. 6B shows an autoradiograph of the gel shown in FIG. 6A. FIG. 6B illustrates that two DNA probe strands are necessary for the production of stable protein-free RecA protein catalyzed hybridization complexes.

FIG. 7 illustrates a model of stable double-stranded probe:duplex linear target DNA complexes.

FIG. 8 shows a gel from which stable double-stranded probe:duplex linear target DNA complexes were isolated, where the duplex probe strands were differentially labeled.

FIG. 9 illustrates a variety of double-stranded probe:duplex linear target DNA complexes.

FIGS. 10A, 10B, and 10C illustrate several detection systems based on a single double-stranded probe:duplex linear target DNA complex.

FIGS. 11A and 11B illustrate several detection systems based on a multiple double-stranded probe:duplex linear target DNA complex.

FIG. 12 shows RecA-protein catalyzed two-double D-loop primer positioning on native target DNA (FIG. 12A) and DNA amplification with DNA polymerase followed by ligation with DNA ligase (in the absence of primer/probe displacement) (FIG. 12B).

FIG. 13 shows a DNA polymerase mediated signal amplification reaction using a single double D-loop probe (FIG. 13A) or multiple double D-loop probes (FIG. 13B). In FIG. 13, X and X' can be the same or different: for example X can be radioactively labeled and X' can carry a digoxigenin moiety.

FIG. 14 illustrates a detection system involving the use of restriction endonuclease cleavage of non-target complexed double-stranded probe where capture of the resulting product is accomplished before (FIG. 14B) or after (FIG. 14A) restriction enzyme digestion.

FIG. 15 illustrates the protection of a restriction site by either methylation or RecA protein. In the case of methylation protection the double D-loop complex is deproteinized before restriction endonuclease digestion (FIG. 15A). In the case of RecA protein protection the double D-loop complex is deproteinized after restriction endonuclease digestion (FIG. 15B).

DETAILED DESCRIPTION OF THE INVENTION

I. Generation of RecA Catalyzed Probe:Target Hybridization Complexes Which are Stable to Deproteinization.

A. DNA Probes and Primers

Experiments performed in support of the present invention show that short double-stranded DNA molecules or complementary single-stranded molecules can be used to generate hybridization complexes with linear target DNA molecules at internal regions and these complexes are stable to deproteinization. As an example of these stable hybridization complexes, double-stranded and complementary single-stranded DNA molecules of varying lengths were prepared for use as probes and primers (Example 1, Table 1). These DNA molecules were chosen to have homology to various portions of a 500 bp region of the lambda phage genome. The relationships of the probes and primers, listed in Table 1, to the lambda genome is illustrated in FIG. 1. The nucleotide sequence of the 500 bp lambda genomic region is presented in FIG. 2.

B. Preparation of RecA Protein and Probe Coating

In the present invention RecA protein refers to a family of RecA-like recombination proteins all having essentially all or most of the same functions, particularly: (i) the protein's ability to properly position primers on their homologous targets for subsequent extension by DNA polymerases; (ii) the ability of RecA protein to topologically prepare DNA for DNA synthesis; and, (iii) the ability of RecA protein/DNA primer complexes to efficiently find and bind to complementary sequences. The best characterized RecA protein is from E. coli; in addition to the wild-type protein a number of mutant RecA-like proteins have been identified (e.g. RecA803). Further, many organisms have RecA-like strand-transfer proteins (e.g., Fugisawa, H., et al.; Hsieh, P., et al., 1986; Hsieh, P., et al., 1989; Fishel, R. A., et al.; Cassuto, E., et al.; Ganea, D., et al.; Moore, S. P., et al.; Keene, K., et al.; Kimiec, E. B., 1984; Kimeic, E. B., 1986; Kolodner, R., et al.; Sugino, A., et al.; Halbrook, J., et al.; Eisen, A., et al.; McCarthy, J., et al., Lowenhaupt, K., et al.)

RecA protein is typically obtained from bacterial strains that overproduce the protein: Example 2 describes the purification of wild-type E. coli RecA protein and mutant RecA803 protein from such strains. Alternatively, RecA protein can also be purchased from, for example, Pharmacia (Piscataway, N.J.).

The conditions used to coat DNA probes with RecA protein and ATPγS are described in Example 3. Alternatively, probes can be coated using GTPγS, mixes of ATPγS and rATP, or rATP alone in the presence of a rATP regenerating system (Boerhinger Mannheim).

The coating of probes with RecA protein can be evaluated in a number of ways. First, protein binding to DNA can be examined using band-shift gel assays (McEntee et al.). Example 3 describes the use of the DNA band-shift gel assay to illustrate RecA protein binding to DNA probes. Labelled probes were coated with RecA protein in the presence of ATPγS and the products of the coating reactions were separated by agarose gel electrophoresis: FIG. 3 shows an autoradiogram of the resulting DNA in the gel. The data presented in FIG. 3 illustrates that following incubation of RecA protein with denatured duplex probe DNAs the RecA protein effectively coats single-stranded DNA probes derived from denaturing the duplex probe. As the ratio of RecA protein monomers to nucleotides in the probe increases from 0, 1:27, 1:2.7 to 3.7:1 for 121-mer (lanes 1-4, respectively) and 0, 1:22, 1:2.2 to 4.5:1 for 159-mer (lanes 5-8, respectively), DNA probe's electrophoretic mobility decreases, i.e., is retarded, due to RecA-binding to the DNA probe. The partial retardation of the DNA's probe mobility observed in lanes 2,3 and 6,7 reflects the non-saturation of probe DNA with RecA protein. Thus, as expected (Leahy et al.), an excess of RecA monomers to DNA nucleotides is required for efficient RecA coating of short DNA probes.

A second method for evaluating protein binding to DNA is the use of nitrocellulose filter binding assays (Leahy et al.; Woodbury et al.). The nitrocellulose filter binding method is particularly useful in determining the dissociation-rates for protein:DNA complexes using labeled DNA. In the filter binding assay, DNA:protein complexes are retained on a filter while free DNA passes through the filter. This assay method is more quantitative for dissociation-rate determinations because the separation of DNA:protein complexes from free probe is very rapid.

Typically, to perform such filter binding assays nitrocellulose disks (Schleicher and Schuell, BA85 filters or HAW P0025 nitrocellulose filters) are pretreated, soaked in buffer and then placed on a vacuum filter apparatus. DNA:protein binding reactions are often diluted to reduce the concentration of components without dissociating complexes. The reactions are passed through the discs with vacuum applied. Under low salt conditions the DNA:protein complex sticks to the filter while free DNA passes through. The discs are placed in scintillation counting fluid (New England Nuclear, National Diagnostics, Inc.), and the cpm determined using a scintillation counter.

C. DNA Targets

To study the specificity of the RecA catalyzed hybridization of probes with homologous double-stranded linear DNA targets at internal sites, several model lambda DNA target systems were used including the following:

(1) A mixture of 13 DNA fragments, ranging in size from 92 to 8598 bp generated by DraI (Promega) restriction enzyme digestion of complete lambda genomic DNA (48.5 kb; Bethesda Research Laboratories, Gaithersburg, Md.). The target fragment homologous to the region defined in FIGS. 1 and 2 is a 8370 bp DraI fragment. The region(s) of homology to the probes, listed in Table 1, were all at least 832 bases in from the 3' end of the double-stranded target DNA fragment.

(2) A mixture of two fragments (38,412 and 10,090 bp) generated by ApaI restriction enzyme digestion of complete lambda genomic DNA. The approximately 10 kb fragment contains the region defined in FIGS. 1 and 2. This region of homology lies at least 2460 bp from the 3' end of the double-stranded target DNA fragment.

(3) The 10 kb fragment of an ApaI lambda DNA digest, agarose gel purified and deproteinized.

(4) A double digest of lambda with DraI and BamHI in which the target DNA fragment is 2957 bp, with probe target homology at least 832 bases from the 3' end of the double-stranded target DNA fragment.

(5) Whole lambda viral DNA was also used as target. In this case, probe:target homology was at least 7131 bases from the 5' end of the whole lambda genome.

D. RecA-Facilitated Formation of Hybridization Complexes Between Double-stranded Probe and Target DNA Sequences

The mixing of RecA coated single-stranded DNA probes and target DNA initiates the search for homology between RecA coated DNA probes and duplex target DNA molecules. In the case of a single probe sequence, once the RecA:DNA probe filament is formed it can catalyze the search for homology and D-loop formation between complementary probe and target DNA sequences. Traditional single D-loops can be formed between single-stranded RecA-coated DNA probes with about 500 bases or less of homology with linear double-stranded target DNAs. These D-loops are unstable after protein removal when the position of probe:target homology is at an internal position on the linear target.

Experiments performed in support of the present invention have demonstrated that 500-mer and smaller probes can form stable deproteinized RecA catalyzed double D-loop probe:target complexes at internal sites on duplex linear DNA targets. However, to form such stable structures, at least two probes must be used that have overlapping complementary sequences to each other. The two probes are RecA coated single-stranded DNA probes and are used in RecA catalyzed probe:target hybridization reactions.

Example 4 describes the formation of RecA protein-mediated double D-loops, or multiplexes. The 500- and 280-mer probes were RecA protein-coated and the target DNA was the 8369 bp lambda DNA DraI fragment described above. The RecA protein to probe-nucleotide ratio was 1.5:1 for 500-mer and 1.3:1 for 280-mer. The double-stranded DNA probe to homologous double-stranded DNA target fragment ratio was 11:1 for 500-mer and 22:1 for 280-mer. FIG. 4A shows an ethidium bromide stained DNA gel on which the DNA components of the deproteinized hybridization reactions were resolved.

FIG. 4B shows an autoradiograph of the DNA in the gel shown in FIG. 4A. The results presented in FIG. 4B show formation of 500-mer:target and 280-mer:target DNA hybridization products that are stable to deproteinization. A comparison between the reactions with and without RecA protein, shows that RecA is required for the formation of homologous probe:target DNA complexes.

The pUC18 double-stranded circular DNA was included as a positive control in Example 4. Negatively supercoiled double-stranded DNA circular molecules are known to form probe:target RecA protein catalyzed hybridization products with short single-stranded probes that are stable to deproteinization (Rigas et al.; and Cheng et al., 1988).

In order to confirm the identity of the hybridization products formed in the above experiment, RecA protein coated probes were reacted with target fragments derived from DraI/BamHI double digest of lambda genomic DNA. In this experiment, the homologous target fragment generated by the double digest was 2957 bp in length and the position of probe:target sequence homology was unchanged from the previous experiments (i.e., 832 bs from the 3' end of the homologous target fragment). The hybridization reactions were performed under identical conditions to those just described for the 8370 bp lambda DNA DraI target fragment.

Electrophoretic separation followed by autoradiographic analysis of these RecA protein catalyzed hybridization reactions showed that deproteinized probe:target DNA complexes now migrated to the position of the 2957 bp target fragment, confirming that the probe:target DNA hybridization reaction was indeed with specific homologous targets.

The RecA protein catalyzed probe:target reactions were carried out in the presence of an excess of non-homologous linear DNA target molecules.

Example 5 describes the formation of complexes stable to deproteinization between small double-stranded probes and linear double-stranded target DNAs. In the hybridization reactions presented in Example 5, denatured probes were coated at a RecA protein to probe-nucleotide ratio of 1.8:1, 0, 5.7:1, 5.9:1, 2.6:1 and lanes 1-6, respectively in FIGS. 5A and 5B. The double-stranded probe to double-stranded target fragment ratios were 4.8:1 (280-mer), 3.6:1 (121-mer), and 5.2:1 (79-mer). FIG. 5A shows DNA from an ethidium bromide stained gel on which the components of the deproteinized hybridization reactions were resolved.

FIG. 5B shows an autoradiograph of the gel shown in FIG. 5A. The results presented in FIG. 5B show that the stable deproteinized hybridization probe:target product can be formed with probes shorter than 280 bases in size. The addition of too much RecA protein appears to decrease the amount of stable product formed in the DNA hybridization reaction (compare lanes 4, 5 and 6). Because the 121-mer and 79-mer probes used in this experiment were derived from restriction enzyme digestion of ³² P end-labeled 280-mer and 500-mer duplex probes, each DNA probe contained either the 5' strand or the 3' strand labeled, not both, as with the 280-mer: the 5' and 3' ends of molecules are identified with respect to whole lambda DNA. The signals observed in lanes 3-6 of FIG. 5B show that either the 5' or the 3' probe strand can take part in the probe:target reaction: this observation is consistent with the conclusion that both probe strands are involved in the formation of the probe:target DNA hybridization complex that is stable to deproteinization.

Numerous hybridization experiments following the basic protocol described in Example 4 and 5 have confirmed that the RecA protein catalyzed hybridization reaction can occur under a broad range of reaction conditions. These conditions can be summarized as follows:

(i) ATPγS concentrations between 1 and 12 mM were tested in probe RecA-coating reactions. This concentration range of ATPγS gave stable hybridization products after target addition: the preferred range was about 2.4 to 8 mM. GTPγS, mixes of ATPγS and rATP, and rATP alone also work in probe coating reactions.

(ii) Mg⁺⁺ acetate concentrations in the final reaction containing the probe and target DNAs worked over a broad range of Mg⁺⁺ concentrations: 4 to 25 mM, with the preferred range being about 6 to 8 mM; (iii) RecA protein concentrations between 17 to 41 mM were tested in probe coating reactions: each concentration was active;

(iv) RecA protein to probe-nucleotide ratios during probe coating between 1:3 and 6:1 were effective with the preferred range being between about 2:1 and 4:1 ratios;

(v) Final (micromolar) double-stranded DNA probe to double-stranded DNA target molecule ratios were between 2:1 and 22:1 all yielded stable deproteinized probe:target hybrids;

(vi) The DNA hybridization reaction works in an analogous Tris-HCl reaction buffer (pH 7.5), although probe coating and strand transfer in acetate buffer appears to give more products than the Tris system;

(vii) DNA probes were effectively coated with RecA protein in the presence of a mixture of ATPγS and rATP, preferred mixtures contained about 1.4 and 1 mM of each component, respectively. The hybridization reaction was inhibited by an excess of rATP in the absence of an ATP regenerating system;

(viii) RecA803 mutant protein was active in forming stable hybridization complexes;

(ix) The hybridization reaction functions in the presence of single-strand binding (SSB) protein (Morrical et al.);

(x) RecA protein coated single-stranded DNA probes, including mixtures of coated-denatured-double-stranded probes, stored at -20° C. for several days were active in hybridization complex formation after incubation with target at 37° C.;

(xi) The hybridization reaction can be carried out with whole lambda genomic DNA as target;

(xii) The region of complementary overlap between the probe strands typically is about 79 base pairs and less than about 500 base pairs. Probes with this degree of complementary overlap form stable products at internal target sites in the RecA-catalyzed hybridization reaction of the present invention. Generation of stable hybridization products was also demonstrated at the ends of linear molecules (for example, using 80-mer probes and the 500-mer duplex as target (FIG. 1)). Standard probe-strand to target-strand complementarity is between 90-100%. However, RecA protein is known to catalyze the formation of hybridization complexes containing some non-specific base pair interactions (Cheng et al. 1989). Accordingly, probe:target complementarity can be reduced depending on probe size and the required specificity of the detection reaction: typically complementarity is not lower than 70% base pair matches between each probe-strand and target-strand.

(xiii) Probes having less than about 79 base pairs of overlap can be used in the present invention: stabalization of the double D-loop, subsequent to deproteinization, may be advantageous when probes of these smaller sizes are used. One method of further stabilizing the double D-loop complexes is psoralen cross-linking (Cheng et al., 1988): such cross-linking is particularly useful in situ since it permits the use of harsh washing conditions.

The results presented in FIG. 5B indicated that the observed probe:target products were stable to deproteinization because both DNA probe strands were present on the same target molecule. One representation of such a stable complex is shown in FIG. 7. This structure is referred to herein as a double D-loop or multiplex DNA structure as opposed to the traditional single D-loop, or triple-stranded displacement loop or triplex structure (two target strands and a single DNA probe complementary to a particular single target strand).

Example 6 presents data that confirms that two RecA protein-coated DNA probe strands are required for the production of stable protein-free probe:target hybridization products on a linear target DNA molecule at an internal region of DNA homology. Individual 121-mer probe strands were chemically synthesized to insure that individual probe strands would not be contaminated with small amounts of the complementary (opposite) DNA strand. In order to distinguish the presence of each of the two individual complementary DNA probe strands, the probes were differentially labeled: one strand with a 5' terminal ³² P label and the other with a single 5' terminal biotin label.

Since only one strand was radioactively labeled the ³² P specific activities of each double D-loop DNA hybridization reaction were the same: accordingly, comparison between the results of all experiments was more convenient. Hybridization reactions were performed as described in Example 6. FIG. 6A shows an ethidium bromide stained gel on which the DNA components of the deproteinized hybridization reactions were resolved.

FIG. 6B shows an autoradiograph of the DNA in the gel shown in FIG. 6A. The results in FIG. 6B show that two probe strands are required for stable deproteinized probe:target hybrid production. In addition, the reaction works whether both probes are coated with RecA protein together or in separate reactions. Further, the hybridization reaction generates protein-free stable complexes even when the DNA probes are added to the reaction sequentially (lanes 5 and 6). The addition of the ³² P strand first to the reaction mix appears to provide more DNA hybridization product. It is possible that the terminal biotin label is slightly inhibitory due to the size of the chemical spacer arm or the position of the label on the probe. However, regardless of the order of probe addition to the hybridization reactions, two probe strands are required to generate protein-free stable homologous complexes.

The rate of RecA-facilitated homologous pairing of short DNA probes to their cognate target sequences has been shown to be positively related to the length of attached heterologous DNA tails (Gonda et al.). Accordingly, probes used in the hybridization reactions of the present invention may include heterologous tails, i.e., terminal sequences that are non-homologous to the target DNA, in order to speed the homologous pairing of the probe sequence to the target sequence.

E. Capture/Detection System.

The presence of both the ³² P- and biotin-labeled 121-mer probe strands on the same target molecule was further confirmed using a capture/detection system (Example 7). The deproteinized double D-loop products were captured using streptavidin-magnetic beads. Capture of the biotin containing probe simultaneously captured the ³² P-labelled probe. FIG. 8 shows the DNA from the gel from which the probe:target complexes were isolated before streptavidin capture of the biotin moiety. The DNA complexes were isolated by extraction from gel fragments corresponding to the expected size of the probe:target complex (Example 7). The extracted DNA was then exposed to streptavidin-coated paramagnetic beads. The beads were then isolated and placed in scintillation fluid for detection of the ³² P-labelled DNA probe strand. The results of this analysis are presented in Table 2. The data show that only reactions using two probe strands and RecA protein give a capture signal above background. This experiment used isolated DNA target migrating at the 10 kb target DNA position for capture, thus ruling out the possible presence of complex recombination products between multiple 10 kb targets that could be captured and detected without actually having a double D-loop structure on an individual 10 kb target molecule.

II. Utility

FIG. 9 shows a number of possible double D-loop structures. FIG. 9A represents the formation of a double D-loop structure at an internal site on a DNA target molecule. FIG. 9B represents a similar structure except that the probe DNA molecules have been tailed with heterologous DNA (Gonda et al.). Such tailing can serve several purposes: (i) facilitating RecA loading onto small probes; (ii) providing an extension molecule for the inclusion of labels in the probe, for example, digoxigenin or biotin; (iii) providing a capture sequence; and (iv) providing a sequence to hybridize to an additional reporter molecule.

FIG. 9C represents the situation where two probes are used that have a region of complementary overlap (i.e., a region in which they are complementary to each other) in addition to homologous terminal extensions (i.e., regions complementary to the target DNA but not to the other probe).

FIG. 9D represents the situation where two probes are used that have a region of complementary overlap in addition to heterologous terminal extensions (i.e., regions not complementary to the target DNA but not to the other probe). FIG. 9F shows a similar situation where heterologous terminal extensions are present at both the 5' and 3' ends of each probe strand. FIG. 9G illustrates the situation where the homologous tails are complementary to each other, but not to the target DNA.

The double D-loop structures need not be composed of only two probes. For example, FIG. 9E shows a double D-loop structure generated from 5 separate probe strands: the internal probe strands have regions of complementary overlap to more than one other probe strand. The total region of complementary overlap is typically 79 to 500 base pairs, but, as discussed above, this region may be smaller.

The structures in FIG. 9 illustrate several, but not all, possible combinations of probe and target DNA that can generate double D-loop structures stable to deproteinization. One common feature of double-stranded probes to be used in double D-loop reactions is a region of complementary overlap between the probe strands.

The ability to form stable RecA protein catalyzed deproteinized double D-loop probe:target complexes at internal sites allows specific identification of homologous linear DNA targets. This double D-loop reaction provides new possibilities for hybridization diagnostics. The assay provides the advantages that differentially labeled complementary probe strands can be used in a single reaction and only one small target sequence needs to be known.

Reassociation of complementary probes is inhibited when saturating levels of RecA protein are used (Bryant et al.). Reassociation of such probes is also reduced by the inclusion of ATPγS as a cofactor in the probe coating reactions.

A. Target DNAs.

The method of the present invention can be used to diagnose infectious disease caused by organisms in clinical samples. These organisms can be diagnosed by the detection of specific DNA characteristic of the causative organism. Such organisms include the following: bacteria, like Salmonella, Neisseria, Chlamydia, Shigella, and Streptomyces; viruses, like Herpes simplex virus-1 (HSV-1), herpes simplex virus-2 (HSV-2), and adenovirus, all double-stranded DNA viruses; parasites, like Plasmodium and Giardia; and mycoplasma, like Mycoplasma pneumonia, M. genitalium and Pneumocystis.

For any diagnostic assay, probe sequences are chosen to a known region of homology in the target DNA. Target DNA can be prepared from a number of different sources by standard techniques: for example, aqueous solutions, mammalian tissue, cultured cells, plant tissue, bacteria, yeast, blood and blood components (Ausubel et al.; Maniatis et al.; Sambrook et al.; Davis et al.; Fey; Strickler et al.; Kingston; Wachsmuth).

In general, the detection methods of the present invention can be applied to the detection of duplex DNA in any nucleic acid sample. Applications other than clinical diagnosis of infectious diseases include (i) screening cultured mammalian cells for the presence of contaminants, such as mycoplasma (Zlvin et al.), (ii) diagnosis of certain genetic diseases caused by specific deletions/mutations, insertions or rearrangements in mammalian DNA, such as α-thalassemia, β-thalassemia, or chronic myelocytic leukemia and (iii) hybridization probes to distinguish the presence or absence of a given target sequence in a duplex DNA molecule.

B. The Use of One Double D-loop Structure for Diagnostic Applications

FIG. 10 illustrates several embodiments of the use of one double D-loop structure for the isolation and identification of corresponding sequences in duplex DNA targets. One probe can be labeled with a capture moiety (e.g., as was done with biotin in Example 7). The other probe is then labeled with a detection moiety, such as a radioactive label, biotin, or digoxigenin or other modified bases. The probes are coated and hybridized to the nucleic acid sample that is being tested for the presence of the target sequence. The hybridization reactions can be deproteinized or used directly.

The probe labeled with the capture moiety is trapped. This trapping can be accomplished by, for example, labeling the probe with a biotin moiety and exposing the reaction mixture to streptavidin that has been attached to a solid support. Alternatively, the capture moiety can be digoxigenin and the trapping can be accomplished using an anti-digoxigenin antibody attached to a solid support. Additional groups can be conveniently attached to the ends of DNA molecules as follows. The oligonucleotide probe is combined with digoxigenin-11-dUTP (an analog of dTTP, 2'-deoxy-uridine-5'-triphosphate, coupled to digoxigenin via an 11-atom spacer arm, Boehringer Mannheim, Indianapolis, Ind.) and terminal deoxynucleotidyl transferase (GIBCO BRL, Gaithersburg, Md.). The number of dig-11-dUTP moieties incorporated using this method appeared to be less than 5 (probably only 1 or 2). Alternatively, dig-11-dUTP moieties can be incorporated into the oligonucleotide sequence of a probe as with biotin.

Typically, the following combinations of double-stranded probes are used in the capture detection system: (i) first probe/capture, labeled with biotin or digoxigenin, second probe/detection, radiolabeled; (ii) first probe/capture, labeled with biotin, second probe/detection, labeled with digoxigenin; or (iii) first probe/capture, labeled with digoxigenin, second probe/detection, labeled with biotin.

One convenient method to sequester captured DNA is the use of streptavidin-conjugated superparamagnetic polystyrene beads as described in Example 7. After capture of DNA, the beads can be retrieved by placing the reaction tubes in a magnetic rack.

Alternatively, avidin-coated agarose beads can be used. Biotinylated agarose beads (immobilized D-biotin, Pierce) are bound to avidin. Avidin, like streptavidin, has four binding sites for biotin. One of these binding sites is used to bind the avidin to the biotin that is coupled to the agarose beads via a 16 atom spacer arm: the other biotin binding sites remain available. The beads are mixed with hybridization complexes to capture biotinylated DNA (Example 7). Alternative methods (Harlow et al.) to the bead capture methods just described include the following streptavidinylated or avidinylated supports: low-protein binding filters, or 96-well plates, or modified biotin capture methods such as iminobiotin (Rigas, B., et al.).

For either of the above bead methods, the beads are isolated and the amount of hybridization complex that has been captured is quantitated. The method of quantitation depends on how the second strand DNA probe has been prepared. If the second probe is radioactively labelled the beads can be counted in a scintillation counter. Alternatively, the captured DNA may be detected using a chemiluminescent, fluorescent or colorimetric detection system.

Many of the experiments described above have made use of radio-labelled oligonucleotides: Example 7 combines the use of a biotin labeled first probe with a radioactively labelled second probe. The techniques involved in radiolabeling of oligonucleotides have been discussed above. A specific activity of 10⁸ cpm per μg DNA is routinely achieved using standard methods (e.g., end-labeling the oligonucleotide with adenosine [γ-³² P]-5' triphosphate and T4 polynucleotide kinase). This level of specific activity allows small amounts of DNA to be measured either by autoradiography of gels or filters exposed to film or by direct counting of sample in scintillation fluid.

Radiolabeling and chemiluminescence (i) are very sensitive, allowing the detection of sub-femtomole quantities of oligonucleotide, and (ii) use well-established techniques. In the case of chemiluminescent detection, protocols have been devised to accommodate the requirements of a mass-screening assay. Non-isotopic DNA detection techniques have principally incorporated alkaline phosphatase as the detectable label given the ability of the enzyme to give a high turnover of substrate to product and the availability of substrates that yield chemiluminescent or colored products.

For chemiluminescent detection, biotinylated or digoxigenin-labelled oligonucleotide probes can be detected using the chemiluminescent detection system "SOUTHERN LIGHTS", developed by Tropix, Inc. The basic technique can be applicable to detect DNA that has been captured on either beads, filters, or in solution.

Alkaline phosphatase is coupled to the captured DNA complex. To do this several methods, derived from commonly used ELISA (Harlow et al.; Pierce, Rockford, Ill.) techniques, can be employed. For example, the second strand DNA probe can be end-labelled with digoxigenin-11-dUTP (dig-11-dUTP) and terminal transferase (as described above). After the DNA is captured and removed from the hybridization mixture, an anti-digoxigenin-alkaline phosphatase conjugated antibody is then reacted (Boehringer Mannheim, Indianapolis, Ind.) with the digoxigenin-containing oligonucleotide. The antigenic digoxigenin moiety is recognized by the antibody-enzyme conjugate.

Captured DNA hybridization products are detected using the alkaline phosphatase-conjugated antibodies to digoxigenin as follows. One chemiluminescent substrate for alkaline phosphatase is 3-(2,-spiroadamantane)-4-methoxy-4-(3'-phosphoryloxy) phenyl-1,2-dioxetane disodium salt (AMPPD). Dephosphorylation of AMPPD results in an unstable compound, which decomposes, releasing a prolonged, steady emission of light at 477 nm. Light measurement is very sensitive and can detect minute quantities of DNA (e.g., 10² -10³ attomoles).

Colorimetric substrates for the alkaline phosphatase system have also been tested. While the colorimetric substrates are useable, use of the light emission system is more sensitive.

An alternative to the above biotin capture system is to use digoxigenin in place of biotin to modify the first strand probe: biotin is then used to replace the digoxigenin moieties in the above described detection system. In this arrangement the anti-digoxigenin antibody is used to capture the DNA hybridization complex. Streptavidin conjugated to alkaline phosphatase is then used to detect the presence of captured oligonucleotides.

Other alternative capture systems include the following: (i) the use of a DNA binding protein and its cognate binding sequence, where the cognate binding sequence is the capture moiety that is included as a 5' terminal sequence in the first strand probe (Kemp et al.); and (ii) the use of hybridization capture where a non-target-complementary DNA sequence, such as poly(T), is incorporated as a 5'terminal sequence in the first strand probe, and a complementary nucleic acid, such as poly(A), is used to capture the probe and associated nucleic acid by hybridization. Either of these two methods can be used in conjunction with a solid support.

Another alternative system is to fix one probe to a membrane (Saiki et al.), coat the probe, add target and the second coated probe, deproteinize, wash and detect.

FIG. 10 illustrates several arrangements for one double D-loop structure detection. FIG. 10A shows one probe strand labeled with a capture moiety, such as biotin, and the second probe strand labeled with a detection moiety, such as digoxigenin. FIG. 10B shows one probe strand labeled with a capture moiety, such as digoxigenin, and the second probe strand having a homologous tail extension that is labeled with multiple detection moieties, such as biotin. FIG. 10C illustrates the addition of detection (or capture moieties) to heterologous tails attached to either and/or both strands of the double-stranded probe.

C. The Use of Multiple Double D-loop Structures for Diagnostic Applications.

In addition to exploiting one double D-loop structure complexes in capture/detection systems, multiple double D-loop structures can be used as well. In cases where reassociation of probe strands may be a problem, for example with larger probes, the use of multiple double D-loop structures provides for a reduced background. In this method two or more sequences need to be known in the target sequence.

FIG. 11 illustrates several arrangements for detection based on two double D-loop structures. FIG. 11A shows an example of labeling both strands of a first duplex probe with the capture moiety and both strands of a second duplex probe with a reporting/detection group. The capture moiety can be contained in either one or both strands of the first probe set. In this system the hybridization complexes are captured as described above, however, reassociation of the strands of the first duplex probe generates no background for the reaction since neither strand contains a reporter/detection group. After capture, the hybridization complexes are detected on the basis of the presence of the second duplex probe in the hybridization complex. Detection of the reporter group is accomplished as described above.

A second embodiment of this system, based on the presence of multiple double D-loop structures in the target complex, is illustrated in FIG. 11B. In this case only one strand of the first duplex probe is labeled with the capture moiety and only one strand of the second duplex probe is labeled with the reporter moiety.

As described above for one double D-loop structure detection, heterologous tails and sequences homologous to the target DNA can be added to the duplex probes.

D. The Use of Double D-loop Structures in RecA Protein Facilitated DNA Amplification Reactions

DNA amplification reactions have been described that predominantly rely on thermal denaturation (Mullis; Mullis et al.; Scharf et al.) for strand separation in preparation for continued amplification. Described below are two detection systems based on DNA amplification subsequent to the formation of single or multiple double D-loops.

(i) One amplification/detection method of the present invention utilizes multiple double D-loop structures to facilitate amplification without the need for thermal denaturation.

Experiments performed in support of the present invention have demonstrated that use of two pairs of complementary DNA primers, which are homologous to different regions of the double-stranded target, in the hybridization reactions of the present invention results in the formation of two double D-loops in the target DNA. These double D-loops flank and define a specific region of DNA on a native duplex target (FIG. 12A): DL801/2 and DL803/4 (Table 1) are examples of probes/primers that can participate in this reaction.

The resulting DNA target structure is recognizable by various DNA polymerases as a substrate for DNA synthesis. An example of one such amplification reaction is presented in Example 8. The substrate for the amplification reaction described in Example 8 is the lambda DNA genome. The primers define a target region of approximately 500 bp. Typically, the amplification reactions contain DNA polymerase (e.g., the Klenow fragment), RecA protein coated primers, ATPγS (or rATP and a suitable regenerating system), the target substrate, necessary co-factors and all four dNTPs (including modified or labeled dNTPs). These reactions may also contain DNA helicase or a similar DNA unwinding agent and/or other DNA polymerases.

These reaction conditions favor extension of the primers at their 3' ends with subsequent primer elongation in the 5' to 3' direction. The 3' end extension of two or more of the four DNA primers in the structure by polymerase defines regions of DNA target amplification between DNA primers (see FIG. 13). DNA polymerase extension of the other two primers can also occur at their 3' ends, unless these are chemically or physically blocked to restrict DNA amplification to a defined region (Example 8).

DNA polymerases catalyzing 3' extension between primer pairs may displace primers on the same strand, or, alternatively, new synthesized product(s) could be ligated to the primer with DNA ligase, including thermoresistant or thermosensitive DNA ligases (Epicentre Technologies, Madison, Wis.). Replication can also be facilitated by adding appropriate DNA helicase, single-strand binding proteins (SSB), gene 32 (or other similar) proteins, or RecBCD-encoded enzymes or other proteins, some of which have associated helicase activities. Any primers properly positioned by RecA protein could be used for replication initiation.

Typically, probes used in the two-double D-loop RecA protein catalyzed DNA amplification are approximately 60 to 80 bp in size. Probes larger or smaller can also be used as well, but reaction conditions may need to be modified, for example, inclusion of stabilizing peptides (e.g., SSB or gene 32 protein), drugs, antibodies, polyamines or cross-linking reagents.

Multiple primers of different sizes can also be used. Primers can also be homologous to the target DNA duplex along their entire length, or they can contain end or internal regions of partial non-homology, such as heterologous tails (see above). The only requirement for these primers is that the bases used for the 3' extension of the desired amplification product is available to prime DNA synthesis. As described above, primers can also contain modified phosphate backbones or bases such as biotin or digoxigenin or appended functions such as DNA modifying enzymes and chemical agents.

Reaction in the presence of excess RecA protein-coated primers allows formation of new multiple or double D-loops on the newly replicated and amplified DNA. The RecA protein-coated primers serve to initiate additional rounds of DNA synthesis, and in this way the DNA target is amplified without the need for thermally denaturing the target DNA to position the amplification primers.

DNA amplification reactions can use any of a number of DNA polymerases or polymerase mixtures, including the following: Klenow large fragment of E. coli DNA polymerase I; T7; T4; and/or other viral or cellular DNA polymerases and their mutants, for example, double-Klenow mutant proteins having no exonuclease activity.

The DNA products of two-double D-loop reactions are defined by the DNA primers used. The left and right primers to the double D-loop regions to be amplified define the 5' ends of the newly synthesized DNA amplification products. When the primers are not displaced, the newly synthesized DNA product can be ligated in a RecA-catalyzed amplification reaction as illustrated in FIG. 12B.

Using multiple primer sets, it is also possible to generate DNA amplification products which have cohesive ends. The DNA products can then hybridize through their overlapping cohesive ends and the length of these associated DNAs is subsequently extended (Haase et al.). Elongation of existing strands, displacement synthesis, and RecA-catalyzed base pairing in the overlap regions may increase the yield of large DNA targets. This can be important for RecA-catalyzed DNA amplification in cells in situ, which may, under certain conditions (Haase et al.), require large DNA products or appended groups for retention in situ.

The double D-loop reaction, or multiple double D-loops, can be stably formed in agarose for in situ amplification reactions. Typically, the agarose is of the low-melting temperature variety, although mixtures of different types of agarose are possible, and the concentration of agarose is about 0.4-1%. Under these conditions the agarose gel provides a restrictive medium that also allows retention of shorter DNA products: this is particularly useful in in situ reactions (see below).

The RecA-facilitated DNA amplification reaction can be carried out at 37° C. as well as at elevated temperatures that are below the thermal denaturation temperatures of target DNA duplex or primer:target hybrids, for example 50°-60° C. Use of elevated temperatures in these reactions expands the repertoire of enzymes available for primer extension and may allow longer tracts of DNA to be synthesized. The temperature of the amplification reaction will dictate the choice of reaction components: for example, wild-type E. coli RecA-protein coated probes are added to the target DNA at 37°-39° C., DNA sythesis is then accomplished at 50°-55° C. with Thermus aquaticus DNA polymerase, the temperature of the reaction is then lowered to 37°-39° C. and RecA-protein coated probes are added re-added. Alternatively, at high temperatures temperature-resistant RecA-like proteins could replace the wild type E. coli RecA protein (as discussed in co-pending, co-owned, U.S. application Ser. No. 07/520,321).

The two-double D-loop, or multiple double D-loop, reaction using RecA protein-catalyzed primer positioning for DNA amplification reactions has important diagnostic applications for DNA detection and amplification in solution diagnostics or in situ diagnostics.

(ii) A second detection method exploiting the double D-loop and DNA amplification uses polymerase addition of labeled or modified dNTPs for signal amplification. Extension of the 3' end of a primer in a single (triple-stranded) D-loop structure using DNA polymerase was demonstrated by Cheng et al. (1988). After formation of a single double D-loop in a target DNA molecule DNA polymerase, necessary cofactors, and all four dNTPs are added to the reaction. Strand extension takes place from the 3'ends of each strand of the double-stranded probe (FIG. 13A). Alternatively, the 3'-end of the strand containing the capture moiety can be blocked to prevent primer extension. One or more of the dNTPs is labeled for detection by a standard method (e.g., biotin, digoxigenin, or, fluorescent or radioactive moieties). The incorporation of the labeled dNTP results in the amplification of the detection signal.

This method can be further exploited by using multiple double D-loop structures to target the region of interest (as described above). The 3'-ends of the double-stranded primers external to the target region can either be blocked (as illustrated in FIG. 13B by an asterisk) or not. This method of signal amplification can be further enhanced by using multiple rounds of RecA-facilitated amplification, as described above, in the presence of a labeled moiety.

E. The Use of Double D-loop Structures in RecA Protein Facilitated in situ Hybridization.

Another detection method which utilizes the double D-loop structure is in situ hybridization with fixed cells (Example 9): RecA-facilitated in situ hybridization methods have been described in co-owned U.S. Patent Application "in situ Hybridization Method," filed on even date herewith. The in situ hybridization of RecA protein coated duplex probes provides the ability to localize a target sequence in an isolated, fixed biological structure or within a nucleus or nuclear volume relative to other targeted sequences and/or the nuclear membrane, using a confocal laser scanning microscope (van Dekken et al.).

One application of the in situ method is described in Example 9D. In this method, dividing HEp-2 nuclei are fixed and probed with the RecA/chromosome-X alpha satellite DNA probe complex, and labeled with FITC-avidin. The pattern of probe binding in the dividing nucleus is evaluated using standard light fluorescent or laser scanning microscopic techniques. To localize the bound probe, the same field is viewed by phase contrast microscopy, without changing the focus of the lens. By overlaying the resulting two photomicrographs, the relative position of the nuclear membrane and nuclear division plane can be seen with respect to the probe-labeled chromosomes.

This aspect of the present invention provides simplified in situ procedures for localizing target sequence(s) in a biological structure. Typically, fixed cells or subcellular structures are probed in suspension or on slides followed by flow-cytometric or microscopic analysis. The method reduces artifacts by eliminating the need for a heat denaturation step, and allows more rapid and specific detection of target sequences. The method can be applied equally well to unique, low, and/or high-copy number target sequences.

In particular, the method allows detection of low-copy sequences without the requirement to first amplify the sequences. Since most gene mapping and chromosomal studies are expected to involve specific, unique, or low-copy sequences, the present in situ method provides an important advantage for gene mapping studies, as well as for diagnostic applications involving unique or low-copy numbers of normal, mutant or pathogen sequences. Also, the present method allows for determination of chromosome content by flow cytometric analysis.

One general diagnostic application of this in situ method is for use in mapping a selected gene or regulatory sequence in a chromosome, and/or in a particular region of the chromosome. The target gene may be one which (a) generates a selected gene product, (b) is suspected of performing a critical cell-control function, such as a cell or viral oncogene, (c) is related to a repeat sequence, (d) is suspected of containing a genetic defect which prevents expression of an active gene product, (e) may be related in chromosome position to a marker probe region with a known map position, and/or (f) may represent an integrated viral sequence.

The probe strands for in situ hybridization can be labeled in a number of ways including direct labeling with fluorescent moieties like fluorescein-11-dUTP (Boehringer-Mannheim). Further, individual probe strands can be used to generate a coupled-fluorescence system where, for example, the emission energy of one fluorescent moiety, incorporated in one strand) emits light at the excitation energy of the second fluorescent moiety, incorporated in the second probe strand. Such a coupled-fluorescence system takes advantage of the proximity of the probe strands in the double D-loop complex.

When the DNA probes are directed against specific cellular pathogens, typically for detecting the presence of a viral or bacterial pathogen in an infected cell, the fixed labeled cells may be examined by light or fluorescence microscopy to detect and localize infecting pathogens in cells. Alternatively, cell infection, and percent cells infected, can be determined by fluorescence activated cell sorting (FACS) after in situ hybridization of RecA protein coated duplex probes to nuclei or cells in suspension (Trask et al.).

F. The Use of Double D-loop Structures in Restriction Enzyme Cleavage Based Detection Systems

The double D-loop structures of the present invention can be used to detect the presence of target DNA in a sample by introducing alterations at the target/double D-loop complex which modify, in a detectable manner, the response of this complex to restriction enzyme digestion. Several examples of such detection systems are described below.

One method of detection that exploits the double D-loop and restriction enzyme digestion is as follows. A region in the target DNA is chosen as the double-stranded probe sequence. The probe sequence is modified to contain an internal restriction site that is not present in the target DNA. Such a restriction site can be chosen so as to minimize base pair mismatching between the target and the probe (FIG. 14). The double D-loop is then formed and the complexes deproteinized. The complexes are then captured on the solid support and digested with the restriction enzyme for which the site has been introduced in the probe sequence (in FIG. 14A, PvuII). Alternatively, the complexes can be digested with the restriction endonuclease before capture (FIG. 14B). Since the PvuII restriction site is not reconstituted when the probe is hybridized to the target sequence, the restriction enzyme will only cleave renatured probe:probe complexes, not probe:target complexes. The solid support is washed and examined for the presence of the detection moiety. This method allows the reduction of any background signal that may be generated by probe renaturation.

A second detection method works on a similar principle. In this method the target DNA is un-methylated and the double-stranded probe DNA is methylated before RecA protein-coating. The double D-loop complex is formed, the complex captured and deproteinized. The sample is then digested with, for example, DpnI which cleaves its recognition site (SEQ ID NO:2) only when the A residue on both strands is methylated. Since the methylated restriction site is not formed when the probe hybridizes to the target sequence, DpnI cleavage only occurs when the probes are renatured. The solid support is washed and examined for the presence of the detection moiety. As above, this method also allows the reduction of any background signal generated by probe renaturation.

The methylation state of the DNA can also be exploited using the target/double D-loop complexes as follows. In this method either the target DNA is methylated or the double-stranded probe is methylated prior to RecA protein-coating. The double D-loop complex is formed, the complex captured and deproteinized. The captured complexes can be isolated from the solid support and split into multiple samples. One sample is digested with a methylase-sensitive or methylase-dependent restriction enzyme and another is digested with a methylase-insensitive restriction enzyme: for example, MboI does not cleave DNA when the A residue is methylated and Sau3A I cleaves the same restriction site independent of the A residue's methylation state.

These samples can then be size fractionated (e.g., on an agarose or acrylamide gel, or by HPLC) and the banding pattern of the samples compared. This method allows isolation and subsequent examination of restriction fragment length polymorphisms of a chosen fragment between a number of samples from different sources.

Methylation may also be used to protect a specific restriction enzyme site from digestion (Nelson et al.). If, for example, it was desirable to isolated a MboI fragment spanning a particular region, but internal to the region an MboI site existed, the fragment could be isolated as follows. A double D-loop structure is formed at the internal restriction site in the target DNA using methylated probes. The target/double D-loop complex is deproteinized, digested with MboI and captured (FIG. 15A). This method can be used to (i) examine restriction enzyme polymorphisms at restriction sites adjacent to the protected site in fragments obtained from different sources, or (ii) capture and clone a desired sequence using a restriction enzyme even when an internal cleavage site is present for that enzyme.

The double-stranded RecA protein-coated probes can themselves be used to protect a specific restriction enzyme site from digestion. In this case, the complex is not deproteinized. The target/double D-loop complex is digested with the restriction endonuclease and captured (FIG. 15B).

The ability to block restriction site cleavage is useful in genomic mapping. For example, if the target DNA defines a known region in the genome, like the PvuI site at approximately 26.3 kb on the lambda genome, the known site is protected by one of the approaches described above. Unprotected and protected lambda genomic DNA is digested with PvuI. The change in restriction fragment patterns between the two digests allows one to deduce which fragments are next to one another--based on the disappearance of bands and the sizes of the new bands in the sample containing blocked restriction sites.

Restriction enzymes, and their response to methylation states, are commonly available and conditions for their use are well known in the art (Ausubel et al.; Maniatis et al.).

G. The Use of Double D-loops to Generate Site-Specific Cleavage in DNA

Oligonucleotides have been used to direct cutting agents to specific single-stranded and double-stranded nucleic acids sites (Corey et al.; Dreyer, G. B. et al.; Moser, et al.). One advantage of oligonucleotide directed cleavage is that the experimenter is no longer dependent on existing cleavage functions: any desired DNA cleavage function can be tailor-made. The RecA protein-coated double-stranded probes of the present invention can be used to generate site specific cleavage in a number of ways. For example, a specific target sequence is selected and a double-stranded DNA probe corresponding to the selected sequence is generated. An EDTA moiety is attached to one or both strands of the oligonucleotide probe. One method of attachment of EDTA to an oligodeoxynucleotide via the C-5 of thymidine has been described by Dreyer et al. The probe can then be RecA protein-coated and the double D-loop complex formed with target DNA. Cleavage occurs in the presence of oxygen upon addition of Fe(II) and a reducing agent (usually DTT) to the EDTA-probe:target hybrids.

Alternatively, cutting can be accomplished using peptide fragments derived from DNA binding/cleaving proteins (Sluka, J. P., et al.) which are attached to the oligonucleotide probes. Further, restriction endonucleases that have frequent cut sites or relatively non-specific phosphodiesterases, such as staphylococcal nuclease, can be attached to an oligonucleotide to generate a hybrid catalytic agent that has increased sequence specificity (Corey et al.).

Oligonucleotides are attached to the phosphodiesterase or other cleaving agent either before RecA protein-coating or after double D-loop complex formation and deproteinization. After association of the phosphodiesterase, nuclease or peptide with the double D-loop complex, the reaction conditions are modified to allow the hydrolysis of both target DNA strands in a site-specific fashion. Depending on the activity of the catalytic agent either one or both strands of the double-stranded probe is modified to contain the agent.

H. Use of the Double D-loop in DNA Enrichment.

The double D-loop hybridization complex of the present invention can also be used for separation and enrichment of selected target DNA sequences. For example, the double-stranded probe can be formed containing a capture moiety in one or both of the probe strands. The double D-loop complex is formed between the double-stranded probe and target DNA contained in a mixture of DNA. The double D-loop complexes that contain the probe and target sequence are then separated from the reaction mixture using the capture moiety, by, for example, attachment to a solid support. The complex can then be denatured to release the target duplex from the support and the released DNA renatured to regenerate the original target duplex DNA. Alternatively, the entire double D-loop complex may simply be released from the solid support.

Target DNA duplexes obtained by this method can be used in DNA amplification reactions (Mullis; Mullis et al.) or in standard cloning techniques (Ausubel et al.; Maniatis et al.).

The following examples illustrate, but in no way are intended to limit the present invention.

Materials and Methods

Restriction enzymes were obtained from Boehringer Mannheim (Indianapolis, Ind.) or New England Biolabs (Beverly, Mass.) and were used as per the manufacturer's directions.

Generally, oligonucleotides were radiolabeled with [γ-³² P]ATP and T4 polynucleotide kinase. Labelling reactions were performed in the buffers and by the methods recommended by the manufacturers (New England Biolabs, Beverly, Mass.; Bethesda Research Laboratories, Gaithersburg, Md.; or Boehringer/Mannheim, Indianapolis, Ind.). Oligonucleotides were separated from buffer and unincorporated triphosphates using Nensorb 20 pre-formed columns (NEN-DuPont, Boston, Mass.) as per manufacturer's instructions, and subsequently dialyzed versus dd H₂ O, if necessary.

EXAMPLE 1 Preparation of RecA-coated Probes

A series of double- and single-stranded DNA probes and primers have been generated. The positions of these probes and primers relative to a contiguous 500 base pair region of the lambda phage genome are shown in FIG. 1: the nucleotide sequence of this region of the lambda genome is presented in FIG. 2. The base positions of the 5' and 3' ends of each probe and primer, relative to the lambda viral genome, are listed in Table 1.

                  TABLE 1                                                          ______________________________________                                         LAMBDA BASES DEFINING PROBE AND PRIMER                                         DNA SEQUENCES                                                                  Probe or     Bases Included                                                                             Size                                                  Primer       Sequence*   (bs) or (bp)                                          ______________________________________                                         PCR01*       7131-7155   25                                                    PCR02**      7606-7630   25                                                    PCR03A*      7351-7390   40                                                    DL80-1*      7131-7210   80                                                    DL80-2**     7131-7210   80                                                    DL80-3*      7551-7630   80                                                    DL80-4**     7551-7630   80                                                    500(ds)      7131-7630   500                                                   280(ds)      7351-7630   280                                                   159(ds)      7472-7630   159                                                   121(ds)      7351-7471   121                                                   Biotin-121*  7351-7471   121                                                   121-.sup.32 P**                                                                             7351-7471   121                                                   79(ds)       7552-7630   79                                                    Biotin-79*   7552-7630   79                                                    ______________________________________                                          *5' strand.                                                                    **Opposite strand.                                                       

                  TABLE 2                                                          ______________________________________                                         BEAD CAPTURE OF PROBE:TARGET HYBRIDS SHOWS                                     THAT THE RecA-CATALYZED DOUBLE D-LOOP                                          PRODUCT CONTAINS TWO DNA PROBE STRANDS                                                              .sup.32 P Radioactive DNA                                         Probe Strand(s)                                                                             Counts Captured                                                                  .sup.32 P (Radio-                                                                      % Total Corrected                                             Biotin   active  Counts per                                                                             % of                                    Reac-         (Capture Reporter                                                                               Minute  Counts per                              tion  RecA    DNA)     DNA)    Expected*                                                                              Minute.sup.                             ______________________________________                                         1     -       +        +       4       0                                       2     +       -        +       11      0                                       3     +       +        -       0       0                                       4     +       +        +       110     98                                      ______________________________________                                          *See FIG. 8 for explanation of sample reactions. Percentages were              calculated from the .sup.32 P counts remaining on the Dynal                    streptavidincoated beads after three washes of each capture reaction with      1X acetate reaction buffer. The total expected .sup.32 P counts per minut      were determined by scintillation counting of DNA in minced gel slices fro      experiments identical to those in FIG. 8. Since no radioactive DNA was         added to reaction 3 (containing biotin capture probe only), no radioactiv      counts were expected for this reaction.                                        .sup. Radioactive .sup.32 P counts in DNA were corrected for nonspecific       bead capture of background DNA counts (i.e., counts from reaction 2            without biotin capture probe).                                           

Primers PCR01 and PCR02 correspond to the primers supplied in the "GENEAMP" DNA Amplification Reagent Kit (Perkin Elmer Cetus, Norwalk, Conn.).

Single-stranded primers (PCR01, PCR02 and PCR03A) and single-stranded probes (DL80-1 through 4, biotin-121, 121-³² P, and biotin-79) were chemically synthesized using commercially available phosphoramidite precursors on an Applied Biosystems 380B DNA synthesizer (Applied Biosystems, Foster City, Calif.).

DNA molecules were biotinylated by reaction with a biotin phosphoramidite at the last 5' base (New England Nuclear-DuPont, Boston, Mass.) before deblocking. All chemically synthesized DNA molecules were deblocked according to the manufacturer's specifications.

Short DNA probes (25-mers) were used without further purification. Single-stranded 80-mer and 121-mer DNA probes were purified by polyacrylamide gel electrophoresis using 8%, and 5% or 8% polyacrylamide gels, respectively. Full sized DNA products were obtained by excising DNA bands from the gels that corresponded to the correct size DNA molecule. The DNA molecules were recovered from gel pieces by electroelution using the "ELUTRAP" system (Schleicher and Schuell, Keene, N.H.). Both probes and primers were concentrated by standard ethanol precipitation (Maniatis et al.). Probe and primer DNA concentrations were determined based on UV absorbance at 260nm of the diluted DNA.

Double-stranded 500 and 280 bp regions of the lambda genome (FIG. 1) were synthesized using primers PCR01 and PCR02, or PCR03A and PCR02, respectively, Taq polymerase and standard DNA amplification reaction conditions (Perkin Elmer Cetus; Mullis; Mullis et al.). The amplification products were separated from the DNA primers by electrophoresis through a 0.7% agarose (Sigma Type II, Sigma, St. Louis, Mo.) gel (500-mer) or a 4% "NUSIEVE" (FMC BioProducts, Rockland, Me.) agarose gel (280-mer). The DNA molecules in the bands corresponding to the amplification products were electroeluted, concentrated, and their actual concentrations' determined as described above.

Double-stranded 121- and 159-mer probes were obtained by restriction digestion of purified 280-mer using the enzyme AluI (New England Biolabs, Beverly, Mass.). The DNA probes were isolated by gel electrophoresis, electroeluted and concentrated as above.

Double-stranded 79-mer probe was obtained from restriction digestion of the purified 500-mer using the enzyme HpaII. The digestion products were separated and purified from uncut DNA by electrophoresis using either 3% or 4% "NUSIEVE" (FMC Bioproducts) gels or 1% agarose gels. Specific DNA fragments were recovered from the gels as described above.

Single-stranded or double-stranded DNA molecules were 5'-end-labeled (Maniatis et al.) with [γ-³² P]ATP and T4 polynucleotide kinase (Promega, Madison, Wis.). When necessary, the DNA molecules were dephosphorylated with alkaline phosphatase (Boehringer Mannheim, Indianapolis, Ind.) before labelling with T4 polynucleotide kinase. Un-incorporated label was removed using "NENSORB 20" nucleic acid purification columns (NEN-DuPont). The labeled DNA molecules could be further purified by dialysis against sterile double-distilled water followed by concentration by freeze-drying. ³² P-labeled 121-, 159- or 79-mer were also obtained by the appropriate restriction enzyme digestion of ³² P end-labeled 280-mer or 500-mer.

EXAMPLE 2 Purification of the Wild-Type RecA and Mutant RecA 803 Proteins

RecA and RecA803 proteins were isolated from the overproducing strains JC12772 and JC15369 (obtained from A. J. Clark and M. Madiraju). These strains contain the RecA and RecA803 coding sequences on plasmids present at high copy numbers per cell. Analysis of total protein from JC12772 and JC15369 cell extracts by SDS-polyacrylamide gel electrophoresis, under denaturing conditions, showed that the 38,000-dalton RecA or RecA803 protein is the major protein produced in these strains.

RecA and RecA803 proteins were purified by modification of established procedures (Shibata et al., 1981; Griffith et al., 1985) using fast protein liquid chromatography (FPLC) using a hydroxylapatite column obtained as a powder (BioRad) followed by an anion ("MONO Q", Pharmacia) exchange column.

Protein purification was monitored as follows:

(i) identifying the 38,000-dalton RecA protein by SDS-PAGE ("PHASTGEL" system, Pharmacia, Piscataway, N.J.);

(ii) assay of the RecA ssDNA-dependent ATPase activity using [γ-³² P]ATP and single-stranded DNA (Shibata et al., 1981). The products of the reaction were separated using PEI cellulose thin-layer chromatography (EM Science, N.J.): the PEI plates were developed in a solvent of 0.5M LiCl and 0.25M formic acid. Products were detected by autoradiography.

(iii) assay of DNase activity. DNase activity was monitored by incubating the RecA protein samples with a mixture of phiX174 linearized and super-coiled circular double-stranded RF, and circular single-stranded DNAs in RecA strand-transfer buffer (Cheng et al., 1988) for 1 hr at 37° C. DNA nicking and digestion were monitored after deproteinization by visualizing the DNAs with ethidium bromide after agarose gel electrophoresis and comparing the quantities of each DNA type in the RecA incubated samples with those incubated in buffer without RecA. Only RecA protein samples showing no detectable DNase activity were used.

(iv) assay of D-loop activity with 500-mer oligonucleotide probe using a method modified from Cheng et al. (1988).

Silver stained SDS-polyacrylamide gel profiles of the final "MONO-Q"-purified RecA and RecA803 proteins showed a single 38,000-dalton band from each preparation that was free of other cellular polypeptides.

EXAMPLE 3 RecA Protein Coating Reactions

RecA protein coating of probes was normally carried out in a standard 1× RecA coating reaction buffer (10× RecA reaction buffer: 100 mM Tris acetate (pH 7.5 at 37° C.), 20 mM magnesium acetate, 500 mM sodium acetate, 10 mM DTT and 50% glycerol (Cheng et al. 1988). All of the probes, whether double-stranded or single-stranded, were denatured before use by heating to 95°-100° C. for five minutes, placed on ice for one minute, and subjected in a Tomy centrifuge to centrifugation (10,000 rpm) at 0° C. for approximately 20 seconds. Denatured probes were added immediately to room temperature RecA coating reaction buffer mixed with ATPγS and diluent (double-distilled H₂ O), as necessary.

This reaction mixture typically contained the following components: (i) 2.4 mM ATPγS; and (ii) between 10-40 ng of double-stranded probe. To this mixture either (i) one μl of RecA protein, usually at 5.2 mg/ml (purchased from Pharmacia or purified as described above), or (ii) an equivalent volume of RecA storage buffer (20 mM Tris-HCl pH 7.5, 0.1 mM EDTA, 1.0 mM DTT, and 50% glycerol) was rapidly added and mixed. The final reaction volume for RecA coating of probe was usually about 10 μl. RecA coating of probe was initiated by incubating probe-RecA mixtures at 37° C. for 10 min.

RecA protein concentrations in coating reactions varied depending upon probe size and the amount of added probe: RecA protein concentration was typically in the range of 6.8 to 51 μM. When single-stranded DNA probes were coated with RecA, independently of their complementary probe strands, the concentrations of ATPγS and RecA protein were each reduced to one-half of the concentrations used with double-stranded probes: that is, the RecA protein and ATPγS concentration ratios were kept constant for a given concentration of individual probe strands.

FIG. 3 shows an autoradiogram illustrating RecA protein binding to 121-mer and 159-mer DNA probes as measured by DNA band-shift assays. Heat denatured ³² P-labeled double-stranded 121-mer and 159-mer DNA probes were reacted with RecA protein as described above. The final RecA-DNA reaction mixtures contained 2.4 mM ATPγS. RecA protein or RecA storage buffer was added to each of four reactions containing 0.01 μg of either denatured 121- or 159-mer DNA probe. The final concentration of RecA in each reaction Was 0, 0.137, 1.37 or 13.7 μM in lanes 1 and 5, 2 and 6, 3 and 7 or 4 and 8, respectively (FIG. 3). All RecA/DNA probe coating reactions were performed in a final volume of 10 μl. RecA binding was initiated by incubating all the reactions at 37° C. for 10 min. Five μl aliquots of each reaction were loaded into a 2% agarose gel in 1× TBE buffer and electrophoresed at 9.2 v/cm for 2 hours. A HaeIII digest of φX-174 DNA (GIBCO-BRL, Gaithersburg, Md.) served as a double-stranded DNA size marker (M). Marker DNA was 5' end-labeled with ³² P as described above. The gel was air dried on saran wrap in a Bioscycler oven at 65° C. The dried gel was then exposed to X-ray film.

As can be seen from FIG. 3, retardation of the electrophoretic mobility of the DNA probes increases with increasing RecA concentration.

The same conditions as described above were employed for RecA803 protein.

EXAMPLE 4 Formation of RecA Protein-Mediated Multiplexes

Probe coating reactions were performed as described in Example 3. After the coating reactions were complete target DNA was added to each reaction. The target DNA was derived from the lambda viral genome and if restriction enzyme digested, contained DNA fragments homologous to the probe sequence and nonhomologous ones as well. Typically, 0.66-1.5 μg of target DNA was added to each reaction in 1× reaction buffer. The magnesium ion concentration of the total reaction was adjusted to 12 mM by addition of an aliquot of 0.2M magnesium acetate. Final reaction volumes were usually 20 μl after the addition of the target DNA.

Probe target mixtures were incubated at 37° C. to allow RecA catalyzed homologous probe:target reactions. After incubation for 60 minutes the reactions were deproteinized with proteinase K (10 mg/ml) at 37° C. for 15-20 min, followed by the addition of sodium dodecylsulfate (SDS) to a final concentration of 0.5% (w/v). Aliquots of each reaction were loaded into wells of 0.7-1.0% agarose gels after addition of tracking dye (Maniatis et al.). The gels were electrophoresed either at room temperature or at 4° C. The gels were stained with ethidium bromide and DNA molecules visualized by UV light. The gels were photographed using a red filter and Polaroid 667 black and white film.

When radiolabeled DNA probes were utilized in the reactions, the probe:target complexes were detected by autoradiography of either wet or dried agarose gels using either DuPont "CRONEX QUANTA III" or "LIGHTENING PLUS" intensifying screens and Kodak "X-OMAT AR5" film. For signal quantitation, target DNA bands showing signal on autoradiograms were excised from gels, crushed, suspended in scintillation cocktail ("AQUASOL-2", DuPont-NEN, Boston, Mass.), and the radioactivity counted in a Packard 2000 CA Tri-Carb liquid scintillation analyzer.

Double-stranded 280-mer and 500-mer probes (Table 1) were reacted with a double-stranded linear target DNA fragment (8370 bp lambda DraI digest fragment that contains each probe sequence) as described above. Denatured probe DNA molecules were coated with RecA protein in the presence of 2.4 mM ATPγS and 34.2 μM RecA protein. Denatured probes that were not coated with RecA protein were added to the same reaction conditions minus the RecA protein. After incubation for 10 min at 37° C., 0.66 μg of lambda genomic DNA, which had been digested with the restriction enzyme DraI, was added to each probe mixture: the genomic DNA was suspended in 1× reaction buffer with an adjusted Mg⁺⁺ concentration, as described above. The final micromolar ratio of double-stranded probe to homologous double-stranded target fragment was 10.6:1 for 500-mer and 21.6:1 for 280-mer. Incubation of the probe:target reactions were carried out as described above.

The reactions were deproteinized and loaded into a 0.7% agarose gel. The DNA was subjected to electrophoresis to separate the probe and target DNA fragments. A photograph of the ethidium bromide stained DNA in the gel containing these reactions is shown in FIG. 4A. The three reactions on the left side are control reactions using pUC18 double-stranded circular DNA target and RecA coated single-stranded 69-mer probe (these control substrates were provided by B. Johnston, SRI, Menlo Park, Calif.). The center lane contained 1 kb marker DNA (GIBCO-BRL). The four reactions on the right side of the gel were the lambda DraI digest target DNAs.

This gel was dried and exposed to X-ray film as described above. The resulting autoradiogram is shown in FIG. 4B. Both RecA coated 500-mer and 280-mer probes specifically hybridized with the correct DraI lambda DNA digest target fragment. The position of probe:target DNA homology is at least 832 bp from the 3, end of the 8370 bp target fragment. This result demonstrates the formation of 500-mer and 280-mer RecA-facilitated DNA hybridization products that are stable to deproteinization. Generation of these stable DNA complexes requires RecA protein.

EXAMPLE 5 Stable Complex Formation Between Small Double-Stranded Probes and Linear Double-Stranded Target DNAs

This example describes the use of small double-stranded probes to generate complexes with linear double-stranded DNA that are stable to deproteinization.

The following hybridization reactions were carried out as described in Example 4. All RecA protein-coating reaction volumes were 10 μl. Each reaction contained 2.4 mM ATPγS and 20.5 μM RecA, unless otherwise noted. Final reactions contained 1.3 μg DraI digested lambda DNA. The following reaction conditions correspond to lanes in FIGS. 5A and 5B: Lanes 1 and 2, 280-mer probe with and without RecA protein, respectively; lane 3, 121-mer probe with RecA protein; lanes 4-6, 79-mer probe with RecA protein. The reactions in lanes 5 and 6 contained RecA protein concentrations of 8.54 and 41 μM during the RecA probe coating reaction. The reactions were deproteinized and loaded into a 0.7% agarose gel. The gel was subjected to electrophoresis to separate the probe and target DNA fragments.

FIG. 5A shows a photograph of ethidium bromide stained DNA in an agarose gel showing the lambda DraI digest target fragments from the above reactions. FIG. 5B shows an autoradiograph of the DNA in an agarose gel shown in FIG. 5A. The arrows indicate the migration position of the DraI lambda target fragment homologous to the probes used. The results indicate that complexes stable to deproteinization can be achieved with all of the RecA-coated DNA probes: 280-mer, 121-mer, and 79-mer. As above, the stable complexes were formed at least 832 bases from the end of a linear double-stranded DNA target molecule.

EXAMPLE 6 Stable Complex Formation Requires a Double-Stranded Probe A. Requirement for a Double-Stranded DNA Probe.

RecA-coated DNA probes were used for hybridization with 3.0 μg of lambda DraI digested target DNA per reaction (60 minutes) as described in Example 4. Two chemically synthesized single-stranded DNA 121-mers were used. The strands were complementary to each other. One strand was biotin-labeled and the other ³² P-labeled.

The following reaction numbers correspond to lanes in FIGS. 6A and 6B. Reactions 1, 2, 5, 6 and 3, 4 contained 2.4 mM or 4.8 mM ATPγS, respectively. All reactions used 20 ng of the ³² P-labeled DNA probe strand and 10.4 μg RecA protein: each reaction contained the same ³² P-specific activity. Reactions 3-6 also contained the biotin labeled strand. In reactions 3 and 4, both DNA probes were added to the initial 10 μl RecA reactions at the same time. For reactions 5 and 6, the ³² P- and biotin-labeled probes were each coated with RecA in separate 10 μl reactions, then one-half of each reaction mix was incubated with target DNA for 30 min before addition of the missing complementary RecA coated DNA probe strand. The ³² P-labeled DNA strand was added first to reaction 5 and second to reaction 6.

All reactions were deproteinized with proteinase K and SDS before electrophoresis. FIG. 6A shows a photograph of ethidium bromide stained DNA in an agarose gel showing the lambda DraI digest target fragments from the above reactions. The reaction conditions are summarized in FIGS. 6A and 6B. The autoradiogram of the air dried gel of FIG. 6A is shown in FIG. 6B.

Two DNA probe strands are required for the production of stable protein-free products formed at a homologous internal sequence on linear target DNA (lanes 3, 5, and 6, compare lane 2). RecA protein is also required for product formation (lanes 3, 5 and 6, compare lane 4).

B. A Model of the Double-Stranded Stable Product

FIG. 7 shows a possible model for the deproteinized double-stranded RecA catalyzed hybridization product. FIG. 7 illustrates a stable double D-loop multiplex formed with short end-labeled DNA probes and a double-stranded linear DNA target. The model shown depicts hybridization on the 8370 bp DraI DNA lambda target, where probe:target homology begins at least 832 bases from the short end of the target (3' end with respect to the whole lambda genome). The DraI fragment includes lambda bases 93-8462. The exact regions of homology are defined by Table 1. A single-stranded RecA protein coated probe does not yield complexes that are stable to deproteinization (see FIG. 6B above).

EXAMPLE 7 Probe:Target Capture and DNA Detection

A. Hybridization Reactions.

Complementary 121-mer DNA probes (Table 1) were individually chemically synthesized. The complementary strands were differentially labeled using [γ-³² P]ATP and biotin (the reporter and capture moieties, respectively). All probe coating reaction mixes contained 2.4 mM ATPγS, 20 ng single-stranded probe, 5.2 μg of RecA protein (5.2 μg/μl; Pharmacia), or an equivalent volume of RecA storage buffer (without RecA protein), per 10 μl reaction. For experiments using both biotin- and ³² P-labeled probes (reactions 1 and 4), 10 μl aliquots of the analogous biotin- or ³² P-labeled probe-coating reactions were mixed together (to give 20 μl) before these mixtures were added to the target DNA mix. To keep all reaction volumes constant, reactions with only one single-stranded probe strand (reactions 2 and 3) used 10 μl of probe mix, 10 μl of probe reaction buffer (i.e., no second probe) and 20 μl of target DNA mix.

A lambda target DNA mix was prepared as follows: 1× RecA reaction buffer, 1 to 10 dilution of 0.2M stock Mg acetate, and ApaI digested lambda DNA. Twenty microliters of the target DNA mix was added to each of the 20 μl probe reaction mixtures. These reactions were incubated for 60 minutes at 37° C. The 40 μl reactions were deproteinized, divided into two equal aliquots, the two aliquots loaded in adjacent wells in a 0.7% agarose gel and the components fractionated by electrophoresis (as described above).

The initial specific activity of all reactions containing the ³² P-labeled strand (1, 2 and 4) were identical. The ethidium bromide stained gel with adjacent duplicate lanes of each reaction is shown in FIG. 8. The contents of each reaction are summarized in FIG. 8.

The portion of each lane of the gel corresponding to the 10 kb lambda target DNA (FIG. 8) was excised from the gel. Each excised fragment was placed into a microcentrifuge tube and rapidly frozen using dry ice. The DNA contained in each gel fragment was recovered by squeezing the frozen gel between folded parafilm until no more liquid was extruded. This DNA-containing liquid was then carefully removed with an "EPPENDORF" micropipette.

B. The Capture/Detection Assay.

The presence of two probe strands on the same target molecule (one biotin-labeled and the other ³² P- labeled) was assayed by capturing biotin-containing-probe:target hybrids on streptavidin-coated paramagnetic beads (Dynal, Oslo, Norway). The manufacturer's bead storage buffer was removed before use. The beads were washed in 1× RecA reaction buffer, in 10× RecA reaction buffer, and finally in 1× RecA reaction buffer. Before DNA capture, equal aliquots of washed beads were added to individual 1.5 ml microcentrifuge tubes and the final wash buffer was removed. Liquid was removed from all bead suspensions by placing microcentrifuge tubes containing the bead mixtures in a magnetic separating rack (Promega, Madison, Wis.).

The DNA-containing reaction samples from above were each added to a microcentrifuge tube containing an aliquot of the washed paramagnetic beads. The samples were mixed, and incubated at room temperature for 15 min. Since beads settle with time, the mixtures were shaken several times during incubation to insure efficient biotin:streptavidin exposure. After the capture reaction, i.e., the binding of streptavidin to biotin, the paramagnetic beads in each reaction were amassed with a magnet and the reaction liquid removed.

Each sample of beads was washed three times with 1× RecA reaction buffer. The presence of ³² P-labeled probe strand was assessed by adding liquid scintillation counting fluor to the beads and counting the radioactivity of the DNA captured by each bead reaction. These data are presented in Table 2.

                  TABLE 2                                                          ______________________________________                                         BEAD CAPTURE OF PROBE:TARGET HYBRIDS SHOWS                                     THAT THE RecA-CATALYZED DOUBLE D-LOOP                                          PRODUCT CONTAINS TWO DNA PROBE STRANDS                                                              .sup.32 P Radioactive DNA                                         Probe Strand(s)                                                                             Counts Captured                                                                  .sup.32 P (Radio-                                                                      % Total Correct                                               Biotin   active  Counts per                                                                             % of                                    Reac-         (Capture Reporter                                                                               Minute  Counts per                              tion  RecA    DNA)     DNA)    Expected                                                                               Minute.sup.                             ______________________________________                                         1     -       +        +       4       0                                       2     +       -        +       11      0                                       3     +       +        -       0       0                                       4     +       +        +       110     98                                      ______________________________________                                          .sup. Radioactive .sup.32 P counts in DNA were corrected for nonspecific       bead capture of background DNA counts (i.e., coutns from reaction 2            without biotin capture probe).                                           

In Table 2 percentages were calculated from the ³² P radiolabelled DNA counts minute remaining on the Dynal streptavidin-coated beads after three washes of each capture reaction with 1× acetate reaction buffer. No ³² P-labeled DNA was added to reaction 3, which contains biotin capture probe only: no radioactive DNA counts were expected for this reaction. The "Total Expected Counts" in Table 2 were determined as follows. Identical reactions were performed as described above and the products separated by agarose gel electrophoresis. Gel fragments corresponding to the 10 kb target DNA were excised from the gel, minced, and placed in "AQUASOL". The amount of ³² P-labeled DNA present in the samples was determined by liquid scintillation counting.

The results indicate that the hybridization product, containing two complementary but differentially labeled probes, can be captured using the streptavidin interaction with the biotin labeled probe strand and subsequently detected by a label in the complementary probe strand.

This bead capture of stable probe:target hybrids supports that homologous probe:target complexes catalyzed by RecA protein actually contained two homologous probe strands on the same double-stranded target molecule.

EXAMPLE 8 RecA+ Facilitated DNA Amplification Without Target DNA Denaturation

Reaction conditions for RecA protein facilitated DNA amplification have been described in co-pending and co-owned Ser. No. 07/520,321, for "PROCESS FOR NUCLEIC ACID HYBRIDIZATION AND AMPLIFICATION," filed 7May 1990, herein incorporated by reference.

Double-stranded probe/primer pairs corresponding to DL801/2 and DL803/4 (Table 1) were denatured and coated with RecA protein as described above. To ensure that elongation of DNA primers occurs in only the desired direction, the 3,-ends of the appropriate primers can be terminated by a 2,,3,-dideoxynucleotide. The dideoxynucleotide lacks the 3,-hydroxyl group present in the conventional dNTPs. The absence of the hydroxyl group inhibits extension by preventing the formation of a phosphodiester bond between the dideoxynucleotide and the succeeding conventional dNTP. The addition of the dideoxynucleotide to the primer can be achieved by using the enzyme terminal deoxynucleotide transferase (Pharmacia, Piscataway, N.H.).

The probes are then allowed to react with the target DNA as described above. The product of the above reaction, consisting of two sets of double D-loops, is then used as the substrate in a typical DNA amplification reaction. The DNA reaction can be carried out in buffer containing 10 mM Tris-HCl (pH 7.5), 8-12 mM MgCl₂, and 50 mM NaCl supplemented with 200-750 μM dNTPs and DNA polymerase (e.g., exonuclease-free, DNA polymerase I, Klenow, or T7 DNA polymerase). In addition, the reaction may be supplemented with other enzymes or proteins (e.g. DNA helicase, DNA ligase and SSB protein) which may facilitate the formation of the specific amplification product. The reaction is allowed to proceed for as long as necessary at 37° C. Upon termination, samples could be deproteinized (SDS, Proteinase K and/or phenol extracted) and analyzed by gel electrophoresis. After electrophoretic separation, the resulting amplified DNA can be visualized by either ethidium bromide staining of the DNA in the gel or by DNA hybridization with a target specific DNA probe. Alternatively, amplification DNA probes could be biotinylated and the newly synthesized DNA captured by appropriate means and then reported and detected as previously described.

DNA synthesis reactions are initiated by the addition of 1-2 unit(s) of exonuclease-free E. coli DNA polymerase I (U.S. Biochemicals) and 750 μM of each dNTP. The reactions are maintained at 37° C.

Following the initial addition of polymerase, the reactions can be supplemented with 1 unit of e.g., Klenow and/or additional dNTPs, at specific intervals spaced over the time course of the reaction.

Samples are treated with proteinase K, before being loaded for electrophoretic separation. After electrophoretic separation the resulting amplified DNA fragments can be visualized by either ethidium bromide staining of the gel or by hybridization with a target specific probe.

For hybridization analysis the gel can be transferred by standard protocols (Maniatis et al.) onto hybridization transfer membrane (Hybond-N, Amersham). The DNA is UV cross-linked (Stratolinker, Stratagene) to the membrane. The UV-treated transfer membrane is hybridized with end-labelled (Boehringer Mannheim) probe PCR03A (Table 1): PCR03A (nucleotides 7351 through 7390 of the native lambda genome) is a 40-mer corresponding to an internal DNA sequence of the 500 base pair lambda template that is the target of the above amplification reaction. The membrane is subjected to autoradiography.

EXAMPLE 9 In situ DNA Detection Utilizing the Double D-loop Reactions

A. Preparation of Probe Complex.

Biotinylated chromosome X alpha satellite DNA probe is obtained from ONCOR (Gaithersburg, Md.). Alternatively, probes can be biotinylated by standard nick-translation methods or by polymerase chain reaction (Weier et al., 1990).

The double-stranded probe diluted in sterile ddH₂ O to the desired concentration prior to denaturation, is denatured in a 0.5 ml microcentrifuge tube in a 100° C. heat block for 5 minutes. The tube is immediately placed in an ice water bath for 1 to 2 min followed by a brief centrifugation at 4°-6° C. in a "TOMY" microcentrifuge, and the tubes are returned to an ice water bath. Approximately 5 minutes prior to addition of denatured DNA probe to the hybridization mixture the tube containing the probe is placed in ice in a freezer at -20° C. The probe hybridization mixture contains the following components in a broad range of concentrations and is combined in the order listed: 1 μl of 10× RecA reaction buffer [10× RecA reaction buffer:100 mM Tris acetate pH 7.5 at 37° C., 20 mM magnesium acetate, 500 mM sodium acetate, 10 mM DTT and 50% glycerol (Cheng et al., 1988)]; 1.5 μl ATPγS from 16.2 mM stock) (Pharmacia) (rATP or GTPγS may be used in some reactions); 0.75 μl 20 mM magnesium acetate; 4-60 ng (or more in some reactions) of denatured probe in sterile water; 1.25 μl 0.137 mM stock RecA protein when purchased from Pharmacia [when obtained from other sources or prepared in the laboratory the amount (μl's) added varies according to concentration of stock].

The mixture is incubated at 37° C. for 10 minutes followed by addition of 0.5 μl/reaction of 200 mM magnesium acetate. Final concentrations of reaction components are: 4.0 mM to 10 mM Tris acetate, 2.0 mM to 15 mM magnesium acetate, 20.0 mM to 50 mM sodium acetate, 0.4 mM to 1.0 mM DTT, 2% to 5% glycerol, 1 mM to 2.5 mM ATPγS, 0.005 mM to 0.02 mM RecA protein.

B. Preparation of HEp-2 Fixed Cell Nuclei.

HEp-2 cells were originally derived from human male larynx epidermoid carcinoma tissue. HEp-2 is a chromosome ploidy variable cell line (Chen).

The cells are cultured for 24 hours after seeding in DMEM medium (Whittaker or Gibco-BRL) supplemented with 10% FBS, sodium pyruvate and a penicillin/streptomycin antibiotic mix at 37° C. under standard conditions. Cells are pelleted by low-speed centrifugation and the pellet is resuspended in 75 mM KCl in a 37° C. water bath for between 5 and 15 minutes for the desired amount of nuclear swelling to occur, followed by cell fixation accomplished by the addition of 3:1 ice cold methanol:acetic acid and centrifugation at 6° C.

One ml of fluid is left in the tube with the pelleted cells, additional ice cold methanol:acetic acid is added, and the cells mixed by gentle mixing of the tube, followed by centrifugation. Repeated additions of methanol-acetate degrades cytoplasm (HEp-2 and other cell types may be fixed in alternative ways, some of which do not degrade cytoplasm.)

Preparations of isolated nuclei are fixed by resuspension in 3:1 methanol:acetic acid at a concentration ˜2×10⁶ /ml and is either dropped by pipette in 10 μl aliquots onto clean glass slides which are stored at -20° C., or the suspended nuclei are stored at -20° C. for later use.

C. Hybridization Reactions for Fixed Preparations.

Ten μl of probe mixture/reaction from Example 9A is applied to the fixed preparation on glass slides. Glass coverslips are placed over the hybridization areas and sealed with rubber cement, and reactions are incubated in a moist container in a 37° C. CO₂ incubator for between 1-4 hours.

Following incubation, the rubber cement is manually removed and the slides are washed in coplin jars 3 times for 10 minutes each in 2× SSC (20× SSC: 3M NaCl, 0.3M sodium citrate, pH 7.0 is used in all SSC containing preparations in these assays) in a water bath at 37° C. Other washing conditions may also be employed.

The slides are placed in pre-block solution [4× SSC, 0.1% "TRITON X-100", 5% Carnation nonfat dry milk, 2% normal goat serum (Gibco), 0.02% sodium azide, pH 7.0] for 25 minutes at room temperature (RT), followed by immersion in 5 ug/ml FITC-avidin DCS, cell sorter grade (Vector, A-2011) in preblock solution for 25 minutes at room temperature. The slides are successively washed in 4× SSC, 4× SSC and 0.1% "TRITON X-100", and 4× SSC for 10 minutes each at room temperature, followed by brief rinsing in double-distilled water. The slides are then dried.

Antifade solution is applied to the slides [100 mg p-phenylenediamine dihydrochloride (Sigma P1519) in 10 ml phosphate buffered saline, adjusted to pH 8 with 0.5M carbonate-bicarbonate buffer (0.42 g NaHCO₃ adjusted to pH 9 with NaOH in 10 ml ddH₂ O) added to 90 ml glycerol, and 0.22 um filtered], and coverslips are placed over the preparations. Antifade containing a counterstain such as propidium iodide or DAPI solution can be used instead of antifade alone.

If necessary, signal amplification is performed as follows: Slides are washed for 5-10 minutes in 4× SSC and 0.1% "TRITON X-100" at RT to remove coverslips and antifade, followed by incubation in preblock solution for up to 20 minutes. The slides are then incubated with biotinylated goat anti-avidin antibody (Vector BA-0300) at a concentration of 5 ug/ml diluted in pre-block solution for 30 minutes at 37° C. Slides are successively washed for 10 minutes each in 4× SSC, 4× SSC and 0.1% "TRITON X-100", 4× SSC at RT followed by incubation in pre-block solution for 20 minutes at RT, then immersed in preblock solution with 5 ug/ml FITC-avidin for 20 minutes at RT. Slides are again washed in the 4× SSC series, briefly rinsed in ddH₂ O, and on slides mounted with an antifade or antifade with counterstain.

Specific signals are detected using standard fluorescence microscopy observation techniques.

D. Detection of Specific Chromosome Sequences in Fixed Nuclei and Whole Cells.

The hybridization mixture is combined in the following order: 1 μl 10× RecA reaction buffer, 1.5 μl ATSγS (16.2 mM stock, Pharmacia), 0.75 μl magnesium acetate (20 mM stock), 12 μl (Example 8A) containing 20 to 60 or more ng of denatured probe in ddH₂ O, RecA (0.137 mM stock, Pharmacia). The mixture is incubated in a 37° C. water bath for 10 minutes followed by addition of 0.5 μl 200 mM magnesium acetate.

HEp-2 cell nuclei or whole cells prepared and fixed as described above are stored fixed in methanol:acetic acid 3:1 (or other appropriate fixation solutions) at -20° C. at a concentration of approximately 2-3×10⁶ /ml. About 0.5 ml of the suspended nuclei, approximately 1-1.5×10⁶ nuclei, (or whole cells) are centrifuged in a "TOMY" centrifuge set at 3 to 6 in a 0.5 or 1.5 ml microcentrifuge tube. The nuclei or whole cells are resuspended, sequentially, in 200 μl to 1 ml of 70%, 85% and 100% ice cold EtOH. After the final centrifugation and removal of 100% EtOH supernatant the pellet is resuspended in 200-500 μl 1× RecA reaction buffer in a 0.5 ml microcentrifuge tube, at room temperature, and centrifuged.

The completed probe mixture is mixed with the pellet, and the tube is placed in a 37° C. water bath for 1.5-2.5 hours. Incubation is stopped by addition of 250 μl of 2× SSC at 37° C. followed by centrifugation. The pellet is resuspended in 250 μl of 37° C. 2× SSC and incubated for 5 minutes at 37° C. Following centrifugation the pellet is resuspended in 500 μl blocking solution at room temperature for 20 minutes, then centrifuged and resuspended in 10 ug/ml FITC-avidin in 100 μl blocking solution at room temperature for 20 minutes. The tube is centrifuged and 250 μl 4× SSC mixed with the pellet, again centrifuged, and 250 μl 4× SSC with 0.1 % "TRITON X-100" mixed with the pellet, and again centrifuged with 250 μl 4× SCC all at room temperature. In some instances different concentrations of and/or different washing components are used. After a final centrifugation the pellet was mixed with approximately 20 μl antifade. The prepared nuclei are mounted on a slide and specific signal is detected using standard fluorescence microscopy techniques.

While the invention has been described with reference to specific methods and embodiments, it will be appreciated that various modifications and changes may be made without departing from the invention.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 2                                                   (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 500 base pairs                                                     (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (vi) ORIGINAL SOURCE:                                                          (A) ORGANISM: LAMBDA                                                           (viii) POSITION IN GENOME:                                                     (A) CHROMOSOME/SEGMENT: 500 BASE PAIRS                                         (B) MAP POSITION: 7131 TO 7630                                                 (C) UNITS: bp                                                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        GATGAGTTCGTGTCCGTACAACTGGCGTAATCATGGCCCTTCGGGGCCATTGTTTCTCTG60                 TGGAGGAGTCCATGACGA AAGATGAACTGATTGCCCGTCTCCGCTCGCTGGGTGAACAAC120               TGAACCGTGATGTCAGCCTGACGGGGACGAAAGAAGAACTGGCGCTCCGTGTGGCAGAGC180                TGAAAGAGGAGCTTGATGACACGGATGAAACTGCCGGTCAGGACACCCCTCTCAGCCGGG 240               AAAATGTGCTGACCGGACATGAAAATGAGGTGGGATCAGCGCAGCCGGATACCGTGATTC300                TGGATACGTCTGAACTGGTCACGGTCGTGGCACTGGTGAAGCTGCATACTGATGCACTTC360                ACGCCACGCGGGATGAACCTGTGGCATTTGTGCTGCCGGG AACGGCGTTTCGTGTCTCTG420               CCGGTGTGGCAGCCGAAATGACAGAGCGCGGCCTGGCCAGAATGCAATAACGGGAGGCGC480                TGTGGCTGATTTCGATAACC500                                                        (2) INFORMATION FOR SEQ ID NO:2:                                               (i ) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 4 base pairs                                                       (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (vi) ORIGINAL SOURCE:                                                          (C) INDIVIDUAL ISOLATE: Cleavage site for DpnI                                 (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        GATC 4                                                                     

It is claimed:
 1. A diagnostic method for detecting a linear duplex DNA analyte, having first and second strands, containing a first internal DNA target sequence, comprisingproviding a set of two DNA probes, having first and second probe strands, where the first and second probe strands (i) contain complementary sequences to the first and second target sequence strands, and (ii) where these complementary sequences also contain complementary overlap between the probe strands, coating the probes with RecA protein in a RecA protein coating reaction, combining the RecA coated probes with the linear duplex DNA, which contains the target sequence, under conditions that produce a probe:target complex containing the probe strands and both target strands, where said complex is stable to deproteinization, and detecting the presence of the probe DNA in the probe:target complex.
 2. The method of claim 1, wherein the RecA protein is the wild-type protein of Escherichia coli.
 3. The method of claim 1, wherein the RecA protein is the RecA-803 protein of Escherichia coli.
 4. The method of claim 1, where said coating reaction contains the co-factor ATPγS.
 5. The method of claim 1, wherein the region of complementary overlap between the probe strands is at least about 78 base pairs and less than about 500 base pairs.
 6. The method of claim 1, wherein a probe strand contains an end terminal extension of DNA that is not complementary to either target strand.
 7. The method of claim 6, wherein each probe strand contains an end terminal DNA extension and the DNA extensions are complementary to each other.
 8. The method of claim 1, wherein a probe strand contains a radioactive moiety.
 9. The method of claim 1, wherein said detecting is accomplished by deproteinization of the probe:target complex, followed by electrophoretic separation of the probe:target complex from free probe.
 10. The method of claim 1, where the first probe strand is labeled with a capture moiety and the second probe strand is labeled with a detection moiety.
 11. The method of claim 10, where the capture moiety is biotin and capture of the probe:target complex is accomplished using streptavidin.
 12. The method of claim 11, where the streptavidin is bound to a solid support.
 13. The method of claim 11, where the second probe strand contains a detection moiety selected from the group consisting of a radioactive label and digoxigenin.
 14. The method of claim where the detection moiety is digoxigenin and the presence of digoxigenin is detected using an anti-digoxigenin antibody containing a reporter moiety.
 15. The method of claim 1, for detecting restriction fragment length polymorphisms further comprising,protecting a known restriction site using the probe:target complex containing both probe strands and examining the fragmentation pattern resulting from restriction enzyme digestion of the target DNA.
 16. The method of claim 15, wherein said protection is accomplished by digestion with the restriction enzyme before deproteinization of the probe:target complex.
 17. The method of claim 15, wherein said protection is accomplished by methylation of both probe strands prior to coating the two probe strands with RecA protein in the RecA protein coating reaction.
 18. The method of claim 1, where the detection method further includes restriction fragment length polymorphism analysis where cleavage of the target DNA is accomplished by attaching a moiety to one or both probe strands that is capable of cleaving the target DNA, forming the probe:target complex, and creating reaction conditions that allow cleavage of the target DNA by the cleaving moiety.
 19. The method of claim 18, wherein the cleaving moiety is an iron FeII molecule.
 20. The method of claim 18, wherein the cleaving moiety is selected from the group consisting of non-specific phosphodiesterases and restriction endonucleases.
 21. The method of claim 1, where the detection method further includes restriction fragment length polymorphism analysis where cleavage of the target DNA is accomplished by forming the probe:target complex, attaching a moiety that is capable of cleaving the target DNA to one or both probe strands, and creating reaction conditions that allow cleavage of the target DNA by the cleaving moiety.
 22. The method of claim 1, where said detecting is accomplished by DNA polymerase facilitated primer extension from the 3'-ends of each probe strand, wherein the primer extension is performed in the presence of all four dNTPs and one or more dNTP contains a detectable moiety.
 23. The method of claim 1, which further comprises providing a second set of two DNA probes, having first and second probe strands, complementary to a second duplex target sequence, where the first strand of the probe contains sequences complementary to one strand of the second target sequence and the second strand of the probe contains sequences complementary to the other strand of the second target sequence, where (i) these probes also have a region of complementary overlap to each other, and (ii) the second set of probes does not hybridize to the first set of probes.
 24. The method of claim 23, where the first probe set is labeled with a capture moiety and the second probe set is labeled with a detection moiety.
 25. The method of claim 24, where the capture moiety is biotin or digoxigenin.
 26. The method of claim 24, where the detection moiety is selected from the group consisting of a radioactive-label, biotin, and digoxigenin.
 27. The method of claim 24, where said detecting includes the use of a capture system that traps the probe:target complex.
 28. The method of claim 27, where the capture system involves the biotinylation of one probe set and the capture is accomplished using streptavidin.
 29. The method of claim 28, where the streptavidin is bound to a solid support.
 30. The method of claim 23, wherein the two probe sets define a internal region of the target DNA and each probe set has one strand containing a 3'-end internal to the region and a 3'-end external to the region.
 31. The method of claim 30, where said detecting is accomplished by DNA polymerase facilitated primer extension from the 3'-ends of each probe strand, wherein the primer extension reaction is performed in the presence of all four dNTPs and one or more dNTP contains a detectable moiety.
 32. The method of claim 31, wherein the external 3'-ends of each probe strand are blocked such that primer extension is not possible from the external 3'-ends.
 33. The method of claim 32, where the primer extension reaction is performed below the temperature required for thermal dissociation of the two target strands and continued until a desired degree of amplification of the target sequence is achieved.
 34. The method of claim 33, where said reacting includes further addition of DNA polymerase.
 35. The method of claim 33, where said reacting includes further addition of RecA protein-coated probes. 